FBI to interview Donald Trump as Butler shooting probe continues

While the FBI investigation is focused on Crooks and his motivations - rather than any security failures that took place - Mr Rojek said investigators believe that he arrived at the venue at 13:50 on the day of the shooting, and flew a drone around the area for 11 minutes shortly after.

The FBI believes Crooks left the venue at approximately 16:00 before returning and being identified as a suspicious person shortly after 17:00.

At around 17:30, he was seen using a range finder, according to the FBI's timeline, just under half an hour before he was seen walking near the AGR building with a backpack.

At 18:11, he was confronted by a local police officer, about 30 seconds before he fired eight shots in the direction of Trump and the crowd.

Crooks is now believed to have conducted "early surveillance" of the site on 11 July, two days before the rally.

The newly published text messages extend the time period in which the 20-year-old gunman had provoked suspicion.

Previously reports established that he was on the radar of local law enforcement about an hour before the shooting.

Witnesses told the BBC moments after the shooting they had spotted the gunman on the roof and raised the alarm.

It is still unclear why there was a communications breakdown between local law enforcement and the Secret Service.

Members of the local Swat team told ABC News on Sunday they had no contact with the agency and a face-to-face briefing failed to happen.

On Monday, House Speaker Mike Johnson and Minority Leader Hakeem Jeffries also announced 13 members of a bipartisan task force that will investigate the attempt on Trump's life.

The committee - which is composed of seven Republicans and six democrats - includes Pennsylvania Republican Mike Kelly, whose district includes Butler, and Tennessee's Mark Green, the chairman of the Homeland Security Committee.

Acting Secret Service Director Ronald Rowe and FBI Deputy Director Paul Abbate will appear at a separate hearing before the Senate Homeland Security and Governmental Affairs committee on Tuesday.