The shooter, 20-year-old Thomas Matthew Crooks, had been spotted 30 minutes before the shooting.
Secret Service and local law enforcement each say the other was responsible for securing the building where the shooter in Donald Trump's assassination attempt fired.
Nearly 30 minutes before the shooting, 20-year-old Thomas Matthew Crooks had been spotted acting suspiciously near the metal detectors. He was seen carrying a rangefinder and a backpack.
"The shooter was actually identified as a potential person of suspicion. Units started responding to seek that individual out," Secret Service director Kimberly Cheatle told ABC News. "Unfortunately with the rapid succession of how things unfolded by the time that individual was eventually located they were on the rooftop and were able to fire off at the former president."
Crooks was finally spotted on the rooftop by spectators who raised the alarm two minutes before he opened fire.
Video of Crooks on the roof moments before opening fire was shot by Mike Difrischia.
Difrischia and his wife, Amber, believe the rifle may have been already placed on the roof when Crooks got there.
"Me, her and other witnesses there, we did not see the gun when we were at the bottom of the roof. but when he got to the middle part of the roof that's when everybody noticed he had a gun," Mike says.
Three local police snipers were inside the warehouse when the would-be assassin was about to open fire on the roof above them.
The Secret Service director said no police officers were positioned on the rooftop because it was sloped and was considered unsafe.
It was revealed the 20-year-old gunman was at a shooting range on Friday, the day before the assassination attempt. He bought 50 rounds of ammunition at a gun store and a ladder from Home Depot which he apparently used to reach the rooftop.
The Secret Service said an additional counter-sniper team with the U.S. Secret Service fired one round at the gunman, killing him, according to CBS News.
