tonight, tale of a tape. donald trump and the video he claims he saw despite no evidence that it exists. a new firestorm amid new questions about how melania trump came to america. knife attack an american woman stabbed to death on a london street. a florida college professor's wife. five others wounded amid fears of terrorism. what investigators are now saying. major mob bust. dozens arrested on a laundry list of charges. and the americans to watch. some familiar faces, some new hoping to strike gold here in rio. plus matt lauer with michael phelps. why he says this the eve of the games. "nightly news" begins right now. >> announcer: this is "nbc nightly news" with lester holt reporting tonight from the olympic summer games in rio. >> good evening from copacabana beach, one of the many competition sites for the games starting tomorrow here in rio. in this half of the world, it is mid-winter. even though war u.s. eastern time, the sun has been down here for some time. think february in the states without the cold. we'll have much more on the upcoming competition here later. but we want to start with a snapshot of the race for president. a new nbc news/"wall street journal" poll shows that hillary clinton has opened up a nine-point lead over donald trump. 47% to 38%. that's up from a five-point lead just before the republican convention. and comes as the trump campaign struggles again to regain its controversies. the latest one now involving his wife. nbc's katy tur has details. >> reporter: tonight after his toughest week yet, donald trump is trying to turn the page. training on president obama over a $400 million settlement payment to iran. >> $400 million being flown to iran. i mean, folks, what's going on here? >> reporter: but straying again claiming he saw video of iranians offloading that . us -- has to be -- by the iranians. you know why the tape was given to us? because they want to embarrass our country. >> reporter: the problem? there's no evidence that tape exists. trump repeated his version even after his campaign said he was talking about generic footage of the detainees landing in switzerland. now a new report concerning the timeline around when melania trump and whether it was league for her to work here. newly recovered photos place her in the usa in 1995, a year before she and trump say she first arrived. melania telling msnbc in february -- >> i never thought to stay here without papers. i had visa. >> reporter: tonight firing back blasting inaccurate reporting and misinformation saying she was always in full compliance with immigration laws. >> political spouses are normally off limits but in this case truas immigration, his wife's history was bound to get scrutiny. >> reporter: all of it coming as aides, confidants, and supporters are pleading with trump to get back to basics. are the controversies wearing on you? >> yes, they are. but not as much as they are my wife. >> he has a good message to tell and the democrats don't. >> reporter: the trump campaign is insisting that they are getting back on message. this as new battleground polling trump is down double digits in pennsylvania. lester? >> katy tur in maine, thank you. and for his part, president obama said today that while donald trump needed to receive intelligence briefings as a candidate, a candidate also needed to act like a president and not spread those briefings around. the president also spent a great deal of time talking about the fight against isis. we get more tonight from kristen welker. >> reporter: in a press conference at the pentagon designed to tout the obama strategy, president obama once again jumped into the 2016 race targeting donald trump. >> i obviously have a very strong opinion about the two candidates who are running here. one is very positive and one is not so much. >> reporter: the president today asked about trump's repeated assertions the election will be rigged. >> that's ridiculous. that doesn't make any sense. and i don't think anybody would take that seriously. intelligence briefings which both candidates are now cleared for by law. >> if they want to be present, they got to start acting like president. >> reporter: but donald trump and republicans have also put the president on defense accusing his administration of paying ransom for americans held in iran after revelations the u.s. sent a $400 million cash payment to iran in january coinciding with the release of those americans. >> we have a president who frankly is at his critics arguing the cash was to settle a decades' old financial dispute. >> we do not pay ransom for hostages. >> reporter: kristen welker, nbc news, the white house. we're learning more about a horrific attack on the streets of london. a stabbing spree that claimed the life of an american woman from florida leaving five others injured. investigators say there does not appear to be a terror connection. nbc's kelly cobiella on a central london sidewalk for darlene horton. the wife of a florida state university professor stabbed in the back. a witness who took this picture said paramedics tried for 20 minutes to save her. >> you could just see they were applying a form of cpr. >> reporter: 64-year-old horton died on this sidewalk. tonight fsu's president said there are no words to express our heart ache. her husbandrd london for the summer. it was their last night in the city. the couple due to fly home to tallahassee today. the man injuring five others in a stabbing spree. shown in the video on the ground being restrained was mentally unstable. no ties to terrorism, no sign of radicalization, police said. >> at this time we believe this is a spontaneous attack and that the victims were selected at random. terrorism. you said it was chaos. >> it was chaos. because nobody knew what was going on. >> reporter: tonight praise for the action of police on scene in just six minutes. the suspect quickly spotted, tased, and restrained. sadly too late for a florida family now grieving for a wife and mother. kelly cobiella, nbc news, london. as miami deals with a home grown zika outbreak, the cdc said today that more than now been diagnosed with the virus after catching it overseas. including 479 pregnant women. at his news conference, president obama sharply criticized congress for going on recess before authorizing more funding to fight zika. meanwhile in florida, concern remains high about how far the spread of the virus may go. kerry sanders has the latest. >> reporter: pregnant mothers to be in florida have been yolan baptist among the lucky first. women were told today no testing here, try another office. >> i'm concerned, yes. because this is my first pregnancy and i want it to be a successful one. >> reporter: rather than wait for the state, tracy humphries went to quest diagnostics, a private lab for testing. her husband did too. >> if i do see mosquitoes and i think i have any bites, i would get tested again. >> reporter: zika test results take two to repeated. today president obama criticized congress for failing to fund zika before taking a seven-week vacation. >> zika is a serious threat to americans. especially babies. right now. so once again i want to urge the american people to call their members of congress and tell them to do their job. >> reporter: some good news today. two days now and no more home grown zika cases. the cdc here today clarified the area be as small as 500 square feet. the challenge, making sure those mosquitoes don't bite people who then spread the virus elsewhere. kerry sanders, nbc news, miami. let's turn now to news breaking here in rio. the international olympic committee just announced how many russian athletes will be allowed to compete. a third of the country's team has been banned after a major doping scandal. it comes on the eve of opening ceremony cloaked over a massive security force. the largest ever deployed in brazil. miguel almaguer is with athletes and the military. >> reporter: tonight for team russia in rio, a split decision for many athletes appealing a ban after a widespread doping scandal. >> the message is clear. no place to hide for cheats. >> reporter: boxers and many members of the swim team given the all clear to compete late today. against somebody who has done something that was against the spirit of sports. >> reporter: while rio is iconic for its incredible views, tonight what's most stunning here is security. at the foot of christ, heavily armed soldiers. even along the sands of copacabana, police looking for threats big and small. robbery is the biggest concern down here? >> yes. >> reporter: security remains tight along the olympic torch today few problems in this city of sand and surf. the torch one wave closer to the cauldron as tonight the world watches rio. just 24 hours until opening ceremonies. we expect half a million people to flood beaches like the one behind us here as well as this entire city. lester, with this doping scandal, another layer of drama before the games get under way. the big sporting events, the most >> here we are. thank you very much. the u.s. is represented by 554 athletes here in rio. 292 of them are women. b the most women who have competed for any country. when we look at the athletes to watch, it is the american women leading the way. the usa women's soccer team's 2-0 win over new zealand yesterday sets them on the right path to clinch their fourth gold medal. women. >> in london more american women won gold than american men. women's basketball, they dominate. the williams sisters, they both won four gold medals. they can both win in singles and doubles. >> reporter: i met up with mary carillo to talk about which athletes could make big headlines here. individually, behind us is the swimming venue. we didn't think we'd see michael phelps here. >> no. he already won 22 olympic medals. 18 of them his favorite color, gold. now he's come back and he's got a chance -- he's got a very legitimate chance of taking on maybe three more goals. >> reporter: and don't forget the splash american katie ledecky made four years ago. >> she comes into london as a 15-year-old, wins the 800 free style, stuns the world. >> the future of usa swimming. >> reporter: four years later, she would win a fist full of gold. she wins by jaw breaking margins. >> reporten watchout for simone biles. >> i'm telling you, this kid. she's got real heft to what she does. and she does things that nobody else has ever seen before. >> reporter: and mary says don't count out jamaican sprinter usain both bolt. >> he's as big a presence as anyone. >> reporter: this is olympics 13 for mary who has extra reason to love the rio games. >> i've been watching the candidates saying this is what america represents. i love the idea the american flag is going to go up for the next 17 days and people have relief from all that political fatigue. i don't have to tell you what that's been like. >> reporter: can i hug you and say mary carillo for president? that's my endorsement. mary carillo of nbc olympics with some pretty great perspective there. ahead of tomorrow's official opening of the summer games. still ahead tonight, making sure u.s. olympic athletes stay clean. our exclu being carried out. also why ford has issued a huge recall. back now from rio de janeiro. in the run-up to these games, doping has loomed large. there have been numerous athletes and national teams under the glare of the spotlight. tonight we're taking an ideas look at the u.s. anti-doping agency and why its is the best in the world at detecting cheaters and working to ensure that american athletes are the cleanest. here's nbc's ron mott. >> is there any stopping lance armstrong? >> reporter: the old adage cheaters never win has been debunked time and again in sports. they do win and sometimes get caught. >> the win at all costs culture is across the globe. a mission to make sure american athletes never use drugs to cheat. looming over the rio games, a doping scandal. russia's entire weight lifting team and all but one track and field athlete banned. >> the pressure on athletes to do whatever it takes to stop at nothing in order to achieve success is as great as we've ever seen. >> reporter: here in the u.s., athletes are tested at competitions and away from their sport. it's a system element of surprise. nbc news is with a doping control officer who showed up unannounced at 7:00 a.m. for a sample. >> we don't want the athlete to be prepared. >> reporter: the marathoner answered the door. his 115th usada drug test had just begun. >> you got nothing to worry about. >> reporter: at the olympic trials, after just making the team, a usada official never obtained. >> they make it really easy on the athletes but they also make sure all their t's are crossed and i's are dotted. >> reporter: of the thousands of american athletes tested by usada each year, dozens are sanctioned. more than 120 since 2013. sprinter justin gatlin failed a 2006 drug test and served a reduced penalty in exchange for cooperating. tyson gay was also denied knowingly taking a banned substance. both competing in rio. though his number one priority is leveling the playing field. >> obviously we have to hold individual athletes who cheat accountable. but getting to the root cause, the system of the win at all costs, those are the leaders within the sport. team doctors, trainers, owners who may be pressuring athletes. those are the ones we're going our best to hold accountable. the russians allowed to compete in rio. today the head of the international olympic committee said 10,000 tests will be administered and they're going to keep them on file for ten years for reanalysis. >> all right. ron mott tonight, thank you. when we come back, charges of extortion, federal agents today arrested nearly mafia and running a vast east coast criminal operation in what officials are calling a significant blow to the mob, the fbi rounded up what it said was leaders of the gam bee know and locassi families. the fbi says the families jointly ran an operation in five states involving illegal gambling, fraud, arson, and extortion. ford is recalling more than 800,000 vehicles because the might break and the doors could open while the vehicles are moving. the models involved with 2012 to 2015 and they include certain ford and lincoln suvs, cars, and vans. when we come back tonight, matt lauer with the most decorated american the olympic rings lit here in rio and finally tonight, he is sure to be one of the most talked about athletes of these games. michael phelps taking the plunge for what he says will be the last time. though he looks more or less the same as changed on the inside for michael phelps as he prepares to carry the american flag tomorrow night at the opening ceremony. matt lauer caught up with phelps. >> reporter: from sydney and athens to beijing and london -- >> here comes phelps to the wall! >> reporter: -- michael phelps has won 22 olympic medals. more than anyone else in history. >> and michael phelps is going to win gold! >> reporter: in public he owned the spotlight. but privately fel >> 100% i was lost. i thought of myself as just a swimmer. and nobody else. >> reporter: now 31, the swimmer is a different person than the one we've come to know. >> i'm a lot more laid back, i'm a lot more relaxed, i'm a lot more open. >> reporter: his cheering section is different too. phelps' sisters and mother who've been staples in the stands are now joined by his fiance and their son boomer robert phelps. >> phelps is going to win it. >> reporter: at 2 his son compete for the first time. >> looked like boomer was awake for the race which was good. >> reporter: and qualify for his olympics. >> having my first born be there to watch is something that's special. >> reporter: boomer's middle name is in honor of his coach he's trained with since he was 11. he's like a completely different person. >> absolutely. in all the areas. it's all about the swimming now. into london. >> leading into london, i didn't want anything to do with the sport. >> reporter: in fact, phelps hated swimming. and often wouldn't show up to practice. but he and bowman put on a good front keeping their strained relationship hidden. prior to london, bob, was there a time you didn't think he was going to go? >> yes. >> reporter: you thought he was going to quit? >> i hoped he would. >> reporter: why? >> i thought it was going to end badly. >> reporter: it all second dui and went to rehab. >> i felt like i didn't want to see another day. i felt like it should be over. >> reporter: so did you check yourself into treatment because you had a drinking problem or because you had a public relations problem? >> i checked myself in because i think i was at a point in my life where something needed to change. >> reporter: he's sober now and there's a sense of self-acceptance. >> ready? go! >> reporter: people close to phelps says he hates to lose realistic? >> there's a number in my head. >> reporter: so if you don't hit the number, will you be able to look at me and say i went out the way i wanted to go out? >> yeah. because i'm giving an honest effort. i'm having fun again. and this is something that i haven't had in a really long time. >> reporter: there's no question he's always had that competitive fire. but this time he also has the passion. matt lauer, nbc news, rio. this thursday night from rio, our home for the next couple of weeks.