An investigation is continuing into the assassination attempt on former President Donald Trump, who appeared at the Republican National Convention in Milwaukee Monday night — the first time he has been seen in public since he was shot and injured at his campaign rally in Pennsylvania. 

The crowd at the convention cheered Trump, whose ear was bandaged after it was hit by a bullet on Saturday.

Details are emerging about the moments that led up to the shooting. People alerted law enforcement to the gunman on the roof of a building about 410 feet away from the stage at least two minutes before the first shot was fired at Trump, video analyzed by CBS News shows. 

A local law enforcement officer with direct knowledge of the incident told CBS News three snipers were stationed inside the building the shooter used in his attack. The operations plan had them stationed inside the building looking out windows toward the rally. 

One of the snipers inside saw the gunman, Thomas Matthew Crooks, outside and looking up at the roof, observing the building and disappearing, according to the officer. Crooks came back, sat down and looked at his phone. At that point, one of the snipers took a picture of him. Crooks took out a rangefinder and the sniper radioed to the command post. Crooks disappeared again and then came back a third time with a backpack. The snipers called in with information that he had a backpack and said he was walking toward the back of the building.

Officers believe that Crooks might have used an air conditioning unit to get on top of the roof. By the time other officers came for backup, he had climbed on top of the building and was positioned above and behind the snipers inside the building, the officer said.

Two other officers who heard the sniper’s call tried to get onto the roof. State police started rushing to the scene, but by that time, a Secret Service sniper had already killed Crooks, the officer said.

The FBI is investigating whether the shooter was a politically motivated homegrown domestic violent extremist, and investigators are still combing through his background. Justice Department officials told reporters that investigators have the shooter’s phone and were examining it at the FBI lab in Quantico, Virginia.