WATCH: New footage of Trump shooting shows gunman taking position before attack

A video from James Copenhaver, one of the victims critically wounded in the July 13 assassination attempt against former US President Trump, reveals a figure moving across the roof of the American Glass Research (AGR) building minutes before gunfire erupted at Trump's rally in Butler, Pennsylvania, US, Fox News reports.

The video, captured at 6:08pm on July 13, shows a person on the roof of the building adjacent to where Trump was speaking, visible from the 1:00 mark to about the 2:50 mark. Authorities believe the gunman, Thomas Crooks, 20, began shooting with a collapsible AR-15-style rifle three minutes later, around 6:11 p.m. Counter snipers fatally shot Crooks shortly afterwards, and law enforcement officials found eight shell casings near his body.

In his attempt to assassinate the former president, Crooks killed 50-year-old Corey Comperatore, a husband, father, and former fire chief at the Buffalo Township Volunteer Fire Department. Crooks also critically wounded Copenhaver, 74, and David Dutch, 57. Both victims were shot twice, according to sources familiar with the incident.

Copenhaver's lawyer, Joseph Feldman, stated that Copenhaver was not filming when he was shot. "He had stopped filming when everyone turned their heads to look at a projection screen," Feldman explained.

Copenhaver was discharged from Allegheny General Hospital on 26 July but is still recovering in a rehabilitation centre.

Trump and his rally attendees had turned their heads to look at immigration statistics projected onto a large screen when the first shots rang out. Feldman described Copenhaver's experience: "He had almost seen or heard something whiz past him, which we're assuming was a bullet. He felt it on his arm and looked down… He felt pain initially but hadn’t realised he had been shot a second time. It could have been shock." Pandemonium ensued, and it took Copenhaver some time to realise what had happened to him.

FBI officials explained that Crooks gained access to the roof of the AGR building by climbing HVAC equipment and piping, then traversed multiple rooftops before finding his position. Leading up to the shooting, Crooks researched the distance from which Lee Harvey Oswald shot President John F. Kennedy and visited the rally site on 7 July, spending about 20 minutes in the area.

On the day of the rally, Crooks flew a drone between 3:50 p.m. and 4 p.m., approximately 200 yards from where Trump would be speaking. FBI Director Christopher Wray testified that Crooks had been at the rally site for about 70 minutes the morning of the assassination attempt.

Law enforcement officials were aware of a suspicious person near the rally site approximately 90 minutes before the gunfire began and took photos of Crooks around 4:36 p.m.

The FBI is still working to determine Crooks' motive behind the shooting.