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.

Backing that up is 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 confirmed to 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 New London, Connecticut, police.

The AP also 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.”

“Turning the other cheek when *someone else* is oppressed is not Christian behavior; it is complicity in the oppressor’s crimes,” it later says.

While the manifesto does not reference Donald Trump by name, it alludes to grievances over a range of administration actions and recent events, including US strikes on drug smuggling boats in the eastern Pacific.

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

Allen is believed to have traveled by train from California to Chicago and then onto 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 purchased several weapons from a California gun store and stored them at their parents’ home in Torrance without their knowledge, according to federal officials.