According to multiple outlets ... during Thursday's briefing with U.S. Senators, the Secret Service revealed they spotted shooter Thomas Matthew Crooks on the roof preparing to shoot Donald Trump 20 minutes before he fired the first bullet.

The official timeline, according to ABC News, TMC first came to the Secret Service's attention as a person of interest at 5:10 PM ET on Saturday, more than an hour before the shooting.

At 5:30 PM ET, law enforcement spotted him with a rangefinder camera -- a device that measures the distance between itself and its subject -- a red flag for law enforcement.

Still, Crooks made his way to the fated rooftop ... where Secret Service saw him at 5:52 PM ET -- 10 minutes before DJT took the stage and 20 minutes before Crooks pulled the trigger.

This shocking new revelation adds fuel to the questions about law enforcement's security measures before the shooting, including what seemed like a molasses-like response to the gunman. A person holding a rifle was allowed 20 whole minutes to wait for the former prez and line up his shot seems almost unbelievable.

As we told you ... video shows attendees pointing out Crooks to law enforcement -- who took a long time to respond to the frantic individuals pointing him out. That video was only captured about a minute and a half before the shooting, but clearly, law enforcement had tons more time to respond.

Crooks fired off multiple shots, catching Trump in the right ear before snipers returned fire and took him down.

Secret Service says it only took 26 seconds from when Crooks started shooting to when he was shot dead ... a quick response after the damage was done.

Thomas' behavior leading up to the shooting -- from posts he made on a gaming platform to appearing to lurk around the rally's perimeter -- add to the questions Secret Service Director Kimberly Cheatle, who was already confronted at the RNC, will have to answer. She's been subpoenaed to testify before Congress.