Investigators are looking into anti-Trump sentiment as being a motive for the attacker who sought to breach the White House Correspondents’ Dinner in Washington DC where the US president and top members of his administration were present.

Officials have said that the shooter likely was targeting Donald Trump and other senior administration officials. “We do believe, based upon just a very preliminary start to understanding what happened, that he was targeting members of the administration,” acting US attorney general Todd Blanche said in a TV interview.

The suspect, who is in custody after being subdued by members of law enforcement as he rushed through the hotel venue, is set to be arraigned in federal court on Monday. He faces charges of assault of a federal officer, discharging a firearm and attempting to kill a federal officer, but Blanche said depending on how the investigation goes, he might also be charged with the attempted assassination of Trump.

Investigators are assessing a manifesto reportedly written by the suspect, who has been identified as Cole Tomas Allen, 31, of Torrance, California. In the alleged manifesto, which was published in full by the New York Post, Allen created a list of targets for the shooting, ranked from highest to lowest priority, with Trump administration officials at the top.

An official familiar with the matter told the Guardian that the manifesto published by the Post was authentic.

The suspected gunman sent writings listing his grievances against the administration to his family members about 10 minutes before shots were fired, according to White House officials who spoke to the Associated Press. A relative, confirmed by those officials to be Allen’s brother, contacted police in New London, Connecticut.

Officials have said that the suspect fired a shotgun at a Secret Service agent at a security checkpoint in the Washington Hilton hotel before being tackled and arrested.

Trump, first lady Melania Trump, vice-president JD Vance and cabinet officials were rushed out as the incident unfolded. The Secret Service agent who was shot escaped serious injury because the bullet struck his protective vest, Trump said.

The AP reported that the shooter referred to himself as the “Friendly Federal Assassin”, which matches the text in the manifesto published by the New York Post.

In a statement to the AP, the New London police department said it was contacted at 10.49 pm, about two hours after the shooting, by an individual who wanted to share information related to it. The police department said it then immediately notified federal law enforcement.

Numerous other US outlets also reported unnamed officials saying that Allen’s alleged writings contained anti-Trump sentiments, which echoed the contents of the manifesto published by the New York Post. The Washington Post also reported that the suspect had sent writings to his family along those lines, which matched with claims by Trump to Fox News that a sibling had then contacted the police.

The manifesto published by the Post begins with apologies to those who knew the suspect and lists his motives for the shooting. It said the shooter targeted administration officials with the exception of the FBI director, Kash Patel.

The manifesto also states that Secret Service agents would only be targets if necessary, and that hotel security, Capitol police, and the national guard would all be classified as “not targets if at all possible (aka unless they shoot at me”. Hotel employees and guests were not targets.

“I am a citizen of the United States of America,” the letter states. “What my representatives do reflects on me. And I am no longer willing to permit a pedophile, rapist, and traitor to coat my hands with his crimes.”

He later writes: “I experience rage thinking about everything this administration has done.”

In an interview with 60 Minutes on Sunday, Trump described the attacker as a “sick person”, and accused interviewer Norah O’Donnell of being “a disgrace” for reading out portions of the manifesto on air.

Elsewhere, the presidents described the events of the night in an even tone, saying that he did not feel particularly alarmed as they unfolded.

“I wasn’t worried,” Trump said in the interview when asked how worried he was about possible injuries after hearing the gunshots. “I understand life. We live in a crazy world.”

Allen is believed to have traveled by train from California to Chicago and then on to Washington, where he checked himself in as a guest at the hotel days earlier where the gala dinner was held.

Federal agents have interviewed Allen’s sister in Maryland, who told investigators her brother had legally bought several weapons from a California gun store and stored them at their parents’ home in Torrance without their knowledge, according to federal officials.