Orlando Shooting: A Warning Missed?

Gun store owner says his staff alerted FBI to "suspicious" customer, later recognized as Omar Mateen.
3:20 | 06/17/16

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Transcript for Orlando Shooting: A Warning Missed?
Good evening. And we begin tonight with breaking developments after the terror in Orlando. The president and vice president traveling to Orlando today to meet with the families. We hear from the president just as we also hear from someone else. The owner of a gun store who says they called the FBI weeks before the massacre. They're now convinced the man they were concerned about was omar Mateen. Tonight, they say, the FBI did not visit their store. Also this even, new cell phone video from during the attack. People huddling in a bathroom stall, crying. Comfortable the injured in that bathroom and each other, and trying to stay hidden. Tonight, president Obama saying the country must act, saying the families there in Orlando asking him today, why does this keep happening? In addressing the nation late this afternoon, the president pointing out the children of Newtown would be finishing the fifth grade. We have it all tonight, beginning with those new developments, the possible warning sign missed. And ABC's chief investigative correspondent, Brian Ross. Reporter:he new video captures the fear of that night. Quiet sobbing. And young men and women tried to play dead in a bathroom stall. Comforting each other. Stay with me. Okay. Reporter: After the shooter had already opened fire on them. He just, like, looked through the crack of the stall and just started shooting into the stall, maybe about 60, 70 rounds into the stall. Reporter: Miguel Leiva was in the stall, and he recorded the video, before being shot in the foot. He described how some people then tried to make a break for it. As they were running out of the bathrooms, he was shooting them and killing them right there. Reporter: Throughout the attack, omar Mateen made full use of his years of law enforcement and security guard training. Instructors gave him top scores for marksmanship year after year. He did the training because a lot of his friends were actually police officers. Reporter: But just last year he was denied admission to a Florida police academy, and officials say he claimed it was anti-muslim discrimination. Last month, as Mateen prepared for his attack, employees say he showed up at this Florida gun store, looking for bulk ammunition and body armor, including plates like this one that can stop armor-piercing bullets. The store's owner, Robert Abell, said today that his employees were immediately suspicious about his requests. When he was on his cell phone, he had a conversation in a foreign language, that was more concerning. Reporter: The store refused to sell Mateen anything and called the local FBI immediately after he left. We did contact authorities and let them know, we just had a suspicious person that was in here. And there was nothing that would come of it. And Brian Ross is back with us tonight. Let's, again, repeat what you had said. That the gun store called the FBI, but they claim the FBI never paid a visit. Reporter: That's right, David. The store didn't have Mateen's name, but they did have grainy surveillance video of him. They say the FBI never came to see it, and remember, David, this is someone that's already well-known to the FBI, investigated twice about possible ties to terrorism. So far tonight, in comment from FBI headquarters. So, weeks later, the massacre. Does the surveillance exist anymore. Reporter: It's been recorded over. Brian Ross tonight. Brian, thank you.

This transcript has been automatically generated and may not be 100% accurate.

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