The man accused of attempting to assassinate Donald Trump with a shotgun pleaded not guilty in a DC courthouse on Monday.

Cole Tomas Allen, 31, faces four federal charges stemming from the April 25 shooting at the White House Correspondents' Association Dinner.

This includes attempting to assassinate the president of the United States, which carries a maximum sentence of life in prison. 

He has also been charged with transporting a firearm and ammunition in interstate commerce with intent to commit a felony, discharging a firearm during a crime of violence, and assault on a federal law enforcement officer with a deadly weapon. 

Allen pleaded not guilty on all counts through his lawyer in Washington, DC, federal court. Last month, he broke through a Secret Service checkpoint outside the ballroom where Trump was seated while wielding a shotgun. 

Officers opened fire, discharging five rounds and Allen was ultimately subdued, not by gunfire but after tripping and falling to the ground. 

During the chaotic exchange, prosecutors allege, Allen fired his shotgun at the agents, striking one officer in his bullet-resistant vest. 

The officer was briefly hospitalized but sustained no serious injuries.

Allen's attorneys have called on Acting Attorney General Todd Blanche and DC US Attorney Jeanine Pirro to recuse themselves from the case because they were in attendance at the black-tie event. 

Allen, a former California teacher, was placed on suicide watch and confined to his cell under 24/7 lockdown in the weeks following his arrest at the Washington Hilton.

He sent an email to family members shortly before the shooting, outlining the attack he was about to carry out, according to the DOJ, and signed off with the self-styled nickname 'Friendly Federal Assassin.'

His apparent manifesto also mocked the Secret Service over the event's lax security: 'This level of incompetence is insane.'

Federal prosecutors released a chilling selfie of Allen, pictured in his hotel room, apparently clad with weapons, moments before he charged a security checkpoint in an alleged plot to kill Trump.

Prosecutors say that he took the mirror selfie in his hotel room at 8.03pm in full evening dress just minutes before he arrived at the event with a shotgun.

'He was wearing a black dress shirt, black slacks, and what appears to be a red necktie, tucked into his pants,' the court filing states.

He was also wearing a 'small leather bag' filled with ammunition, a shoulder holster, a sheathed knife, pliers, and wire cutters.

Prosecutors say that after taking the selfie, Allen searched the President’s schedule again at 8.13pm, then looked at live video of Trump arriving at 8.27pm, immediately before his emailed confession went out to family and friends at 8.30pm.

Video footage also provides the clearest look yet at the opening four seconds of the confrontation, with agents firing on Allen and missing every shot.

An officer draws his weapon within two seconds of Allen appearing on camera, then fires multiple times in his direction, with at least some rounds passing close to other agents as Allen runs past them.

After the video emerged, the Secret Service responded by claiming its security measures are 'rigorously tested' and 'were critical in mitigating the threat and preventing significant harm.'