The man accused of opening fire at the White House Correspondents' Dinner sent writings to family members minutes before the shooting referring to himself as a "Friendly Federal Assassin," railing against Trump administration policies and signalling what investigators increasingly believe was a politically driven attack, according to a law enforcement official familiar with the investigation.

The writings, sent shortly before shots were fired at the Washington Hilton, made repeated references to President Donald Trump without naming him directly and alluded to grievances over a range of administration actions and recent events, including US strikes on drug smuggling boats in the eastern Pacific, the official said.

Investigators are treating the writings, along with a trail of social media posts and interviews with family members, as some of the clearest evidence yet of the suspect's mindset and possible motives.

Authorities also uncovered what the official described as numerous anti-Trump social media posts linked to the suspect, Cole Tomas Allen, a 31-year-old California man accused of trying to breach a security checkpoint at the dinner while armed with multiple weapons.

Allen's brother contacted police in New London, Connecticut, after receiving the writings, according to the official. A police spokesperson said they contacted federal law enforcement after receiving that information.

Federal agents have also 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 the official.

She described her brother as prone to making radical statements, the official said.

Allen legally bought a .38-caliber semiautomatic pistol in October 2023 and a 12-gauge shotgun two years later, the official and another law enforcement official told The Associated Press.

Authorities are still trying to determine how specific Allen's alleged targets were. Officials have said investigators are examining whether his grievances centered on Trump and Vice President JD Vance personally or reflected a broader hostility toward the administration.

The officials were not authorised to discuss the ongoing investigation and spoke to AP on condition of anonymity.