The charged with the attempted assassination of US President Donald Trump at a black-tie dinner took a selfie in his hotel room just minutes earlier, wearing an ammunition bag, shoulder gun holster and sheathed knife, court papers show.

Cole Allen wore black trousers, a black shirt and a red tie as he snapped the image in his room at the Washington Hilton, where Trump and hundreds of journalists were meeting for a gala on Saturday night (local time).

Allen, 31, of Torrance, California, was captured when he tried to race past security barricades near the hotel’s ballroom, prompting an exchange of gunfire with Secret Service agents tasked with safeguarding the event, authorities say.

New details emerged on Wednesday in a court filing made by prosecutors, who want Allen to remain in custody. They included included the photo, as well as images of guns and knives that Allen was allegedly carrying on Saturday, to argue he should be detained without bond.

The US government said Allen repeatedly checked online to keep track of Trump that night, including live coverage of the President getting out of his vehicle when he arrived at the Hilton hotel.

Shooting suspect Cole Allen poses with a knife in his hotel room. Photo: US Department of Justice

Investigators said preset emails with an “Apology and Explanation” attachment were sent about 8.30pm.

“[Allen] intended to kill and fired his shotgun while trying to breach security and attack his target,” Assistant US Attorney Charles Jones wrote.

“Put simply, the defendant poses an uncommonly serious danger to the community if released pending trial. The defendant’s lack of criminal history and other personal circumstances do not alter this conclusion.”

Trump was not injured and said a day later he “wasn’t worried” by the incident, which sparked chaos in the Hilton dining room.

A Secret Service officer wearing a bullet-resistant vest was shot in the vest and survived.

Allen appeared in court on Monday and was charged with Trump’s attempted assassination as federal authorities suggested the attack that disrupted one of Washington DC’s glitziest events had been planned for at least several weeks.

He did not speak at length during the quick court appearance, as is customary.

One of Allen’s lawyers, Tezira Abe, noted he had no criminal record and said he was “presumed innocent at this time”.

Trump was rushed off the stage by his security team and appeared at the White House two hours later, still in his tuxedo.

“When you’re impactful, they go after you. When you’re not impactful, they leave you alone,” Trump said.

“They seem to think he was a lone wolf.”

Chaos erupted at the White House Correspondents’ Dinner after Saturday’s shooting. Photos: AAP/TND

An FBI affidavit filed in the case on Monday revealed other details about the planning behind the assault. Authorities allege that Allen reserved his room on April 6 at the Washington DC hotel – weeks before the widely publicised event was due to be held with tight security.

He travelled by train across the US from California last week, checking into the Washington Hilton a day before the dinner with a room reserved for the weekend.

Prosecutors allege that during the trip Allen used his phone to view a news article titled, “Trump’s plans for ‘mic-drop’ media confrontation are leaked: The president is planning a rage-fuelled moment at the White House Correspondents’ Association dinner”.

They also say he kept a running note on his phone about “observations and thoughts during his cross-country train journey”, describing the New Mexico desert, how “Chicago is cool”, and writing that Pennsylvania’s “woods are awesome”.

-with AAP