A remote detonator was found on would-be assassin Thomas Crooks' body alongside his cell phone after he was shot dead by Secret Service snipers at a Donald Trump rally in Pennsylvania on Saturday. 

A picture first published by Pittsburgh's WPXI showed the detonator, a rectangular grey device with a keypad similar to a television remote control, alongside the gunman's cellphone. 

It was earlier reported that investigating FBI agents recovered explosive materials inside of the troubled 20-year-old's car.  

Crooks attracted the attention of security at the rally, held in the town of Butler, 30 miles north of Pittsburgh, after he was behaving strangely around the metal detectors, at least an hour before the attempted assassination, and was later seen using a range finder. 

The gunfire rang out at the rally at 6:11 pm. Trump took to the stage at 6:03 pm, an hour later than advertised. 

Four days after the shooting, law enforcement have not commented on what they believe Crooks may have been planning to do after the shooting with the Secret Service facing increasing scrutiny for their inability to secure the event. 

Channel 11′s Nicole Ford, who obtained the exclusive photo, revealed that law enforcement agents saw Crooks outside of the the single-story AGR International building from which he shot the ex-President. 

This new development comes as Secret Service director Kimberly Cheatle faces fresh questions after local law enforcement officials directly contradicted her claim that a Beaver County sniper team was stationed in the very building from which Trump was shot.

It emerged on Tuesday that the killer penetrated the security area of the rally a full three hours before carrying out his attack, after driving the 50 miles north from his home in Bethel Park on the southern outskirts of Pittsburgh.

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

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 was claimed on Monday that a team of police snipers was stationed in the AGR International building, 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, this 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.

Channel 11 said multiple sources on Tuesday confirmed that Crooks was on the roof with a gun for fewer than seven minutes before he was confronted by a Butler Township police officer.

That officer retreated after Crooks swung his AR-style weapon in his direction leaving the killer free to aim at his target.

Investigators have yet to figure out a motive for Crooks who had a minimal social media presence, and are hoping his phone may hold some clues.

But they found a bulletproof vest, three fully-loaded magazines containing around 100 bullets, and two remote-controlled explosive devices in Crooks' car after the shooting it emerged on Tuesday.

Another bulletproof vest, and third remote-controlled bomb was later discovered at the house he shared with his parents.

The latest revelation that a transmitter was also with him has raised fears that Crooks may have been planning to continue his killing spree if he had escaped from the roof with his life, and may have had an accomplice to help him.

Hours earlier Crooks had asked his boss for the day off, telling him he had 'something to do', before travelling to Saturday's rally.

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

But a Secret Service agent at the Butler rally ensured that he would be the last person Crooks saw as he locked eyes with the killer before shooting him dead, graphic video has revealed.

Agents found him in their sights and one pulled the trigger seconds after the 20-year-old shot dead Pennsylvania fire chief Corey Comperatore, critically injured two others and hit Trump in the right ear from his vantage point 147 yards away.

'They were looking at him while he was looking at them,' a senior federal law enforcement official told CNN.

Video shows Crooks peering intently through the sights of his father's AR-style weapon and firing towards the rally as onlookers scream from the base of the building.

Instantly a second volley of shots is heard and Crooks is pictured lying dead on the roof as those on the ground gasp in horror.

'The Secret Service don't f*** around man,' one exclaims. 'Holy s***.'

He had clambered up building's air conditioning system and hoisted himself onto the roof where MAGA fans saw him 'bear-crawling' across the sloping surface with his rifle, and warned law enforcement of the danger.

'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 2022 high school graduate. Records show he donated on the day of the 2020 inauguration to a progressive group backing President Joe Biden.

He graduated from the Community College of Allegheny County in May with an associate degree in engineering science, according to school officials who said they were 'shocked and saddened by the horrific turn of events'.

Crooks' father Matthew was seen on Tuesday morning for the first time since the shooting as he opened the door of the family home to FBI agents.

A neighbor of Crooks' sister Katie said her brother helped her move into her nearby apartment three years ago, describing her as a 'hardworking' woman who worked as a janitor at a local high school and as a server at a restaurant.

Both of Crooks' parents are behavioral therapists and Crooks' father said he was baffled by his son's actions.

The Justice Department, the Secret Service and the FBI, are due to brief senators about the investigation on Wednesday Majority leader Chuck Schumer revealed on Tuesday night as the Secret Service faces intensive scrutiny over its handling of security at the rally.

Republican officials gathering in Milwaukee have been demanding the agency face accountability even as the feds are implementing tighter security measures around Trump and other protectees.

'Somebody, somewhere has a really, really a lot of serious questions to have to answer,' House Judiciary and 'Weaponization' committee member Kelly Armstrong of North Dakota told DailyMail.com.

A probe set up by President Joe Biden after the incident is being overseen by the Justice Department, not Homeland Security, which oversees the Secret Service.

'That speaks volumes,' former House Oversight Committee Chairman Rep. Jason Chaffetz said.