A team of snipers were inside the building where Donald Trump's would-be assassin climbed onto the roof and opened fire after being spotted 26 minutes earlier, bombshell new reports claim.

Cops at the scene noticed Crooks, 20, clambering into place in plain sight just 130 yards away from the rally stage and took two photos of him because he was acting suspiciously, sources told WPXI.

Meanwhile a counter-sniper team was inside the building that was being used as a 'watch post' during the event when Crooks pulled the trigger, The New York Post reported.  

It's not clear if he had the AR-style rifle on him when he was first seen scaling the AGR International Inc. factory or if he stayed on the roof for the whole time.

The shocking new allegations surfaced as authorities and the U.S. Secret Service face mounting questions over how Crooks was able to shoot the former president and kill a member of the rally crowd.

A horrifying video shows witnesses pointing at the roof and shouting at officers trying to warn them. MAGA fans also say they alerted law enforcement to Crooks as he crawled to his shooting position, but he was still able to shoot.

Law enforcement at the event in Butler, Pennsylvania are facing intense scrutiny after it was revealed the roof was flagged as a security 'vulnerability'.

Secret Service Director Kimberly Cheatle said on Monday night that she would not resign despite mounting calls for her to step aside over the huge lapses that led to one of the most horrific acts of political violence in history. 

The local Pennsylvania sheriff defended the police officer who confronted Crooks on the roof moments before the shooting.

Butler County Sheriff Michael Slupe confirmed on Monday that one of his armed officers encountered the gunman seconds before he unleashed carnage on the former president's rally.

The cop, who has not been named, retreated when Crooks pointed his AR-style rifle at him.

Slupe defended the decision and told KDKA he would 'have done the same thing.'

'All I know is the officer had both hands on the roof to get up on the roof, never made it because the shooter had turned towards the officer, and rightfully and smartly, the officer let go.

'I mean, people think the officers are supermen like you hold on the roof with one hand while you are hanging on for dear life and pull a gun out. It doesn't work that way.

Slupe also said that members of his department were alerted to Crooks' suspicious behavior before the rally began and started searching for him right away.

Local police, not the Secret Service, swept the building, but Crooks was still able to climb to the top and take aim at the former president.

Slupe acknowledged that Crooks was in the 'secondary ring' of protection when he opened fire.

He said the Secret Service is responsible for the ring immediately around the president, but local police take control of the outer ring.

The former president, 78, survived the horrifying assassination attempt when the bullet missed him by less than an inch on Saturday afternoon.

It came eight minutes after he took the stage at 6:03pm ET, where Trump began his remarks and pointed to a large graphic in the same direction where the shooter was perched atop an AGR International building.

Even before he took the stage, Trump supporters said they spotted the gunman 'bear-crawling' across the roof with a rifle, but claimed their warnings to Secret Service were ignored.

'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.'

With the sun still high on a bright day, Trump emerged to 'God Bless the USA' playing over the loudspeakers at 6:03pm, and remained waving and shaking his fist at the crowd for several minutes.

Trump brought out a large graphic with statistics on the migrant crisis at the southern border, and told the crowd at 6:11pm: 'That chart's a couple of months old...'

'If you want to really see something that's sad, take a look at what happened,' Trump continued - before he abruptly grabbed the side of his head as several 'pops' filled the event space.

The 78-year-old dropped to the floor in an instant as five more shots rang overhead in quick succession, before four Secret Service agents raced to the stage and dove on top of the former president.

Stunning images showed Trump cowered on the floor as agents protected him, with blood dripping down his cheek.

Several more Secret Service agents continued to flood the stage, while four heavily armed officers lined the perimeter ordering attendees to 'get down.'

The sniper, sat 400ft across from the rally on an exposed rooftop, fired another shot at the stage a second later while Trump fans streamed out of the bleachers.

 Secret Service scrambled to take control of the pandemonium where three attendees were hit, one fatally, within a matter of seconds.

An emergency room doctor with blood spattered on his shirt later said he sprang into action to perform emergency CPR on one victim.

'The guy had spun around was jammed between the benches and a head shot here. There's lots of blood and he had brain matter,' he told CBS News.

'So I got him together so I got people there really helpful. I did CPR chest compressions as well.'