A gunman who tried to assassinate former president Donald Trump is believed to have flown a drone around the rally site in an apparent attempt to scope it out before the event, a law enforcement official says.

The drone has been recovered by the FBI, which is leading the investigation into the July 13 shooting at the Pennsylvania rally by 20-year-old Thomas Matthew Crooks.

Crooks fired multiple rounds from the roof of a building near where Trump was speaking, clipping the former president’s ear, before being fatally shot by a Secret Service counter sniper.

The existence of the drone and its use at some point before the shooting could help explain why Crooks knew to fire from the point.c

A memo released on Saturday by the Trump campaign and authored by Texas representative Ronny Jackson, who served as the Republican Party nominee’s White House physician, said Trump sustained a gunshot wound to the right ear from a high-powered rifle that came “less than a quarter of an inch from entering his head, and struck the top of his right ear”.

One of the bullets aimed towards Trump killed 50-year-old firefighter Corey Comperatore, a spectator. Two others were seriously wounded.

The FBI is investigating what motivated Crooks.

Investigators who searched his phone found photos of Trump, US President Joe Biden and other senior government officials, and also found he had looked up the dates for the Democratic National Conventional as well as Trump’s appearances. 

More details about the investigation are expected to be made public in the coming week when FBI director Chris Wray appears before the House Judiciary Committee.