States and around the world, im George Howell. Cnn newsroom starts now. Good day. We begin with the efforts of World Leaders to ensure Nuclear Weapons and materials dont fall into the wrong hands. That was the focus of the Nuclear Summit that was held in washington, d. C. The u. S. President Barack Obama Says an important treaty is now expected to go into effect. That treaty requires Member States to do more to secure their Radioactive Materials. Plus, china has pledged to work closer with the u. S. Beijing plays a key role in enforcing sanctions against north korea, which is believed to have Nuclear Weapons. Mr. Obama also says he wants to reduce Americas Nuclear arsenal further, but he says the u. S. Cannot act alone. One of the challenges that were going to have here is that it is very difficult to see huge reductions in our Nuclear Arsenal unless the United States and russia, as the two largest possessors of Nuclear Weapons, are prepared to lead the way. The other area where i think with we need to see progress is pakistan and india, that subcontinent, making sure that as they develop military doctrines, that they are not continually moving in the wrong direction. And we have to take a look at the Korean Peninsula because the bprk, north korea is in a whole different category. And poses the most immediate set of concerns for all of us. One that we are working internationally to focus on. As you heard there from the u. S. President , north korea is still a key obstacle for Global Nuclear peace. It was just thursday that pyongyang fired yet another shortrange missile. This time it landed in the east coast off the peninsula there. All this according to south korea. Plus, north koreas country feels threatened by military exercises between the United States and the south. And now pyongyang says that all the talk in washington will not stop its nuclear program. If the United States continues, then we have to make the countermeasures also, as i told you. So, we have to develop and we have are to make more deterrence, nuclear deterrence. At this point how can you talk about endorsing right now . So we are busy to deal with this, you know, the state us of the cities on the peninsula now. Of all the World Leaders who attended that summit in washington, one major power was missing russia. Our brian todd explains why. Reporter with the brussels and paris attacks, a clear message from isis, they will hit western cities and kill as many civilians as they can. And it appears isis wants to create more devastation. After the paris attackers, investigators in a raid found Surveillance Footage of an belgium so, now we know that they actually are trying to take steps to try and acquire these materials, and so, therefore, we need specific action coming out of this Nuclear Security summit. Reporter but at the Nuclear Security summit in washington, one leader who could make a difference in securing Nuclear Material was a noshow. Vladimir putin has once again snubbed president obama. Russias lack of participation, obviously, in our view, is, frankly, counterproductive. Reporter russia has more than half the worlds stockpile of the worlds Nuclear Materials, and its safeguards havent always been airtight. There was an example several years ago where an insider employee at one of the russian facilities was slowly taking out small quantities of Nuclear Material from that site. Historically, ive been to russia and to their nuclear facilities, in the years after 9 11, it was very am tutor way they were holding these Radioactive Materials. Reporter but experts say the russians have since gotten better at securing Nuclear Material. Security analysts say if putin showed up at the summit, he could have shared intelligence on how to keep isis away from Radioactive Material. Radiological material for a dirty bomb, often stored in hospitals and industrial complexes, is far less secure. Isis can access that. And isis supporters in the u. S. Havent been bashful about where they want to strike. This target is their aspiration, right . Yes. Radiological bomb f it went off here, many people dead in the immediate vicinity, but it would disperse Radioactive Material around several blocks and shut down the city for many years. Reporter responding to the criticism of putin for not showing up at the summit, a russian told us theyre not sending a bad message and theres much more to Nuclear Security than this summit. And some have ticked off reasons for putins absence. Some countries say those with Nuclear Material, like iran, are not participating. They say most of the goals for Nuclear Security have been reached and so no reason to show up. Brian todd, cnn, washington. Americas choice, 2016, the race for the white house and sharp words from the current u. S. President who says dlt trump hasnt shown he knows much about Foreign Policy or much about the world at large, for that matter. Earlier this week the republican president ial candidate suggested that japan and south korea should develop Nuclear Weapons to defend against north korea. They tell us the person who made the statements doesnt know much about Foreign Policy or Nuclear Policy or the Korean Peninsula or the world generally. Meanwhile, a 180 from donald trump. Now back to saying that he could be open to the idea of a third party run. He told fox news it all depends on how Republican Leaders treat him. Are you ruling out running as an independent thirdparty candidate . Are you ruling that out . Im its simple. No, its not that simple. Im by far the frontrunner a republican. I want to run as a republican. I would beat Hillary Clinton. But if you dont get the nomination . Well have to see how i was treated. Well have to see how i was treated, very simple. Back in september the republican candidates pledged to back the partys nominee, but they basically all abandoned that pledge on tuesday, and compounding a tough week for donald trump, some polls show that ted cruz now has a doubledigit lead over him in wisconsin. The next crucial primary. Cnn explains. Do we love wisconsin, right . Do we love it. Reporter donald trump is facing a pivotal moment. Wisconsin is very important. Who is going to vote for trump on april 5th . Reporter tuesdays wisconsin primary now a big test as the gop frontrunner tries to rebound from the toughest week of his campaign. Its a very serious problem. Reporter trump is trying to clean up his comments from earlier this week when asked if women shut be punished for having an abortion f it became illegal . The answer is, there has to be some form of punishment. Reporter now a rare admission from the billionaire. It could be that i misspoke, but this was a long, convoluted subject. Reporter trump is not backing down from another firestorm he sparked this week, standing by his refusal to rule out using Nuclear Weapons in europe. Europes a big place. The last person to use nuclear would be donald trump. I think it is a horrible thing. The thought of it is horrible. But i dont want to take anything off the table. Reporter trump stumbles, providing an opening for the stop trump movement, kicking their efforts into overdrive in wisconsin. Donald trump is coming into a state that i think has his number. Reporter lining up influential conservative talk radio hosts and activists to blunt trump before the race moves east into potentially friendlier territory for trump. I believe he poses an existential threat not just for the Republican Party, but to actually conservative principles and the image going forward. Reporter and trumps rivals are not letting up in their attacks. The problem for him with town halls, he actually has to answer questions in a specific way. Reporter that backlash comes as trump ames to mend fences with the Republican Party. Sitting down thursday with officials at the rnc, including chairman previs. A source familiar with the meeting tells us previs told trump his disparaging comments about the rnc from donors and activists, whose help hell need. Its really a unity meeting. Reporter trump isnt backing down from the campaign fight, releasing a new Instagram Video targeting ted cruz. I think hes been just as wrong as obama, if not worse. Reporter while ted cruz is staying on the offensive against trump. Nominating donald trump could be a train wreck. And thats not fair to the train wrecks. Now to the democratic race and things are getting more personal. Just days ahead of two crucial primaries in the states of wisconsin and new york, frontrunner Hillary Clinton says Bernie Sanders campaign is lying about her record and shoos sick of it. Now sanders says clinton is the one whos lying. So, whos lying . Jason carroll has the latest for us. Reporter a growing intensity in the democratic primary fight as Hillary Clinton tries to shed rival Bernie Sanders. I just go crazy when i hear senator sanders and the Tea Party Republicans railing against the Export Import Bank. Reporter clinton holds a significant lead in the delegate count, but on the heels of a string of western victories, sanders is vowing to carry on to the july convention. If we win here in new york, we are going to make it to the White House Reporter sanders is Challenging Clinton in her adopted home state of new york, rallying more than 18,000 supporters in the bronx thursday night. Here in wisconsin, polls show him with a narrow lead over clinton ahead of tuesdays primary. Despite his delegate gap, sanders insists there is still a path for him to win the nomination. You get 60 , 70 , 08 of the vote in a state. You know what . I think super delegates should vote for us. Reporter sanders is likely to have the resources to push forward. His campaign saying it raised 44 million in march, eclipsing its 43 million haul from february. The hardfought race growing more intense by the day, with clinton being confronted by a climate activist after a new york event thursday night. I am so sick of Sanders Campaign lying about it. If people receive money from lobbists in the industry, i think youre receiving money from the industry. Reporter democratic rivals also sharing jabs over abortion. Clinton accusing sanders of not denouncing trump. Senator sanders agreed that Donald Trumps comments were shameful, but then he said, they were a distraction from, and i quote, a serious discussion about the serious issues facing america. Reporter sanders is charging clinton with misrepresenting his word. What secretary clinton did is take things out of context. I am 100 prochoice. That was cnns Jason Carroll reporting for us. Youre watching cnn newsroom. Still ahead this hour, latest on a deadly overpass collapse in kolkata, india. Who authorities in india are blaming. Plus, kenya marks the somber anniversary of some students who survived the university attack, telling us what they remember from that day. Youre watching cnn newsroom. Im George Howell. The latest on the deadly collapse of an overpass in india. Authorities are now charging a Construction Company for that incident. At least 24 people were killed on thursday in kolkata. An executive of the company denied any fault, saying that the disaster was an action of god. Cnns correspondent has been covering this for us, and she spoke with some of the victims families. Reporter despair and mourning. They were on a handpulled rickshaw headed to a nearby hospital to visit an ailing relative when a meterlong chunk of concrete and metal came crashing down. In seconds their lives ended. While at home the world turned upside down for their two sons. Their shaved heads a sign of grieving in hindu communities. He had to identify his parents. Condition was very bad. They were not in position to see and recognize. It was very bad. Full body was burnt. Reporter their father was the sole breadwinner. He was their only son. We didnt hear from them for hours, we couldnt get in touch with them and then we heard the overpass collapse. I just went cold, she says. After frantic four hours of searching, calling, hoping and praying, she found out what happened. Theres no limit to hardship and sorrow in life. Sometimes its happiness, other times its all darkness. My heart bleeds with pain. He was my only son, she says. In the neighborhood, across the country, people want to know how it happened, who is accountable, but here theres no anger. Who can we blame . We dont blame anyone. We blame our faith. Theyre still in a state of shock, aware of whats happened but unable to make sense of it. Cnn, kolkata, india. In south africa, the countrys president jacob zuma is facing calls to resign. This after south africas highest court determined zuma broke the law in using public funds to renovate his private home. The Constitutional Court ruled mr. Zuma improperly used 15 million in state funds for home upgrades. You see his home here. While some Opposition Party members are urging mr. Zumas impeachment, the ruling Afternoon National congress is rallying around him inspect a televised address, mr. Zuma apologized to south africans. Has caused a lot of frustration and confusion for which i apologize on my behalf and on behalf of government. I urge all parties to respect the judgment and abide by it. Let us use the judgment to build and further strengthen our democracy. President zuma has been ordered to repay state funds that were spent on any upgrades that are not related to security. In kenya, a somber anniversary this day. It has been one year now since the Garissa University massacre. That massacre that killed 148 people, all gunned down by al Shabab Militants. They went door to door on the schools campus, targeting christians specifically. The attack has left a lasting scar on those who survived that day. Many of those students are now attending a school in elduret and thats where our robin creel is following this story. This is a day so many survivors are reliving a nightmare, simply thankful they are alive. This is a day they saw many of their classmates killed. Reporter a truly mixed bag of emotions, george. Every now and then a student walks out in tears. There are 600 Garissa College student here who came here to moore university after their college was attacked last year by shabab gunman. One thing juliette said to me yesterday, shes a survivor we spoke to, is that today could have just as easily been a memorial day for her had she not survived this attack. I think thats whats really resonating with students here, it could have easily been them and not just the Garissa University students who survived. There is a thinking across kenya universities and indeed across the world that this kind of terror attack could have happened anywhere. Heres what a few students we spoke to yesterday had to say about the oneyear anniversary since the garissa attack. Reporter like many University Students, ben is on a journey. Different than most. I crawled up to the door. Thats when i was rescued. And as they were rescuing me, someone from shabab said they were coming in and he shot them. Reporter shot by terrorists on a attack on his university, ben remained silent. One year after suffering debilitating injuries, ben is moving again. Im able to work. Currently i am using a bicycle. I cannot walk for very long distance, but i can run, i can play, i can do very many things. The prayers in the whole and when they shoot us, i fall down. Reporter evelyn can finally walk unassisted and is proud of it. This School Houses almost all of the survivors of the garissa massacre. Hundreds of miles from the state of the attack, its much more secure. Still students fear more attacks and remember the friends they lost. Like judith, she was one of evelyns roommates. Of the six girls in their dorm room, only three survived. Two of them still lived together. Roommate juliette carries one reminder of her best friend. Reminds me of judy. Reporter judith is not here anymore, but her friendship, an everlasting gift. Reporter that singing you hear behind me, george, is the students of Garissa University choir. They are singing a beautiful hymn and they will be heard throughout tod