and i watched in jersey city, new jersey, where thousands and thousands of people were cheering as that building was coming down. >> thousand of people? the problem with donald trump's claim is that it simply is not true. and to date he hasn't provided any evidence to back it up. ♪ in an attempt to back it up trump tweeted out a 2001 article from the "washington post." quote, law enforcement authorities detained and questioned a number of people who were allegedly seen celebrating the attack and holding tailgate style parties on rooftops where they watched the devastation on the other side of the river. "the washington post" went back and re-examined their own reporting and concluded a number of people is not the same as thousands. and allegedly means there's no actual proof. >> we had pockets of celebration. >> reporter: former new york city mayor rudy giuliani