Donald Trump's sho0ter, Thomas Crooks asked his boss for the day off, telling him he had 'something to do', before travelling to the rally in Pennsylvania to carry out his attack against the ex-president.

The would-be assassin told colleagues at the Bethel Park Skilled Nursing and Rehabilitation Center that he would see them on Sunday, July 14.

Agents found him in their sights and one pulled the trigger seconds after the 20-year-old shot de@d Pennsylvania fire chief Corey Comperatore, critically injured two others, and hit Trump in the right ear after scaling a building 147 yards away.

'They were looking at him while he was looking at them,' a senior federal law enforcement official told It emerged on Tuesday that the k!ller penetrated the security area of the rally a full three hours before carrying out his attack, after driving 50 miles north from his home in Bethel Park on the southern outskirts of Pittsburgh.

At 3 pm he triggered a metal detector as he tried to gain access to the site, and was found to be carrying a rangefinder, a g*n sight typically used by hunters and marksmen preparing to shoot at distance.

But security officers allowed him in while keeping an eye on him until he left the secure area a short time later, the official said.

He then fell off the radar until shortly before 5.45pm when an officer with the Beaver County Police Department saw him acting suspiciously near the outer perimeter and took a photograph.

It emerged on Monday that a team of police snipers was stationed in the same AGR International building that Crooks chose to fire from and that they saw him outside three times in the minutes leading up to the attack.

A law enforcement official told CBS that no action was taken after one of the snipers first spotted Crooks looking up at the roof of the building.

The gunman disappeared around a corner before he was seen a second time, sitting down and looking at his phone, prompting one of the snipers to take his picture.

The sniper then radioed to a command post after seeing Crooks take out his rangefinder.

But no further action was taken before Crooks reappeared a third time, wearing a backpack and disappearing from sight as he walked to the back of the building.

A further radio report relaying the information was submitted by the sniper team who did not realize that Crooks was now scaling their building.

'I'm thinking to myself "Why is Trump still speaking, why have they not pulled him off the stage?"' one witness told the BBC. 'The next thing you know, five shots ring out.'

Crooks was a dietary aide at Bethel Park Skilled Nursing and Rehabilitation Center, where he provided food and care for post-hospital elderly and sick people.

'We are shocked and saddened to learn of his involvement as Thomas Matthew Crooks performed his job without concern and his background check was clean,' administrator Marcie Grimm said in a statement.

A colleague at the facility said Crooks never expressed political views at work and wasn't a 'radical'.

'It's hard seeing everything that's going on online because he was a really, really good person that did a really bad thing, and I just wish I knew why,' he added.

The would-be assassin was a registered Republican and a 2022 high school graduate. Records show he donated on the day of the 2020 inauguration to a progressive group backing President Joe Biden.