Donald Trump has agreed to be interviewed by the FBI as part of its investigation into the assassination attempt at his rally in Pennsylvania earlier this month.

In a call with reporters on Monday, the FBI did not provide a date for the interview but stated that it would be “a standard victim interview conducted for any other crime victim.”

“We want to gather his perspective on what he has witnessed,” stated FBI Special Agent Kevin Rojek.

Despite examining mountains of evidence, investigators have yet to determine a motive for Thomas Matthew Crooks’ decision to open fire on 13 July.

Newly released text messages, meanwhile, have revealed that the 20-year-old was spotted by a local SWAT team more than 90 minutes before the shooting, which is much earlier than previously thought.

The messages obtained by the New York Times and ABC News will add to the list of security failures that preceded the assassination attempt against the former president.

On Monday, the FBI said investigators believe Crooks conducted “careful planning” ahead of the 13 July rally and made “significant efforts” to conceal his activities.

That planning – which included six purchases of components for explosive devices – was conducted in a manner that would not “significantly raise the suspicions of his parents”, Mr Rojek said.

Multiple investigations have been launched into what went wrong in securing Butler Fair Show grounds on 13 July.

Secret Service Director Kimberly Cheatle resigned after admitting failures.

On the day of the shooting, the agency was in charge of security within a fenced area and local law enforcement were responsible for areas beyond that.

At 16:19 local time (21:19 BST), the local police sniper texted two colleagues who were in the second floor of a warehouse overlooking the site, telling them he was clocking off.

As he left the building, he saw a young man sitting at a picnic table and notifed the others, saying in a text “someone followed our lead and snuck in and parked by our cars just so you know”.

By 17:38 Crooks had moved from the table to the warehouse, an American Glass Research (AGR) building, and pictures of him were taken and shared in a group chat.

“Kid learning around building we are in. AGR I believe it is. I did see him with a range finder looking towards stage. FYI. If you wanna notify SS snipers to look out. I lost sight of him.”

Other messages obtained by CBS, the BBC’s news partner, showed that at 17:51 a picture of Crooks was forwarded to a local officer, who said that commanders were “asking for a direction of travel”.

About 20 minutes later, Crooks was dead, shot by the Secret Service after opening fire from the roof of an adjoining warehouse.