When David Dempsey was arrested in California on Thursday and charged with attacking police officers defending the US Capitol on 6 January, local activists in Los Angeles were not surprised.

Federal prosecutors have accused 34-year-old Dempsey of striking police at the Capitol with improvised weapons, including a crutch and a pole, and spraying them with a chemical agent, according to the criminal complaint against him.

Five months earlier, two Los Angeles men said, Dempsey had used the exact same tactics to assault them during tense summer political demonstrations in the Tujunga neighborhood and in Beverly Hills.

One of these alleged assaults had happened directly in front of police officers in Beverly Hills, and the other was reported in detail to the Los Angeles police department, according to the two men. Both said that local police failed to follow up or to arrest Dempsey, even though he had previously been charged with using bear mace on anti-Trump protesters in Santa Monica, California, in 2019.

Erik Boyd, a local antifascist activist, alleged that Dempsey punched him and then hit him over the head with a skateboard on 14 August 2020. The skateboard attack left him “bleeding profusely”, and resulted in a deep cut in his head that required at least a dozen stitches, Boyd said.

Boyd said he made a police report about the attack to the Los Angeles police department on 16 August, and provided the Guardian with copies of the emails he sent to an LAPD detective two days later with additional documentation – which included a video of the attack, multiple photographs and videos of Dempsey – and a video of Dempsey allegedly attacking someone else with a skateboard in a similar way that Boyd said he was attacked.

Eight days after Boyd said he was attacked, Sean Beckner-Carmitchel, an independent journalist, said Dempsey and his brother Daniel both attacked him at a protest he was covering in Beverly Hills. Beckner-Carmitchel said he was bear-sprayed in the face, and then grabbed and hit by multiple rightwing protesters as he rolled on the ground. He described it as the most violent attack he has ever experienced.

“This happened in full view of ten police officers,” Beckner-Carmitchel said.

“Not taking my assault seriously in many ways I think contributed to [Dempsey] feeling empowered on 6 January. If you were in jail for assault, I don’t think you would have been there.”

During the attack in Beverly Hills, Beckner-Carmitchel noted, Dempsey appeared to be wearing very similar camouflage pants to the ones he allegedly wore during parts of the insurrection at the Capitol.

In the days after he was attacked, Beckner-Carmitchel said he left phone messages with Beverly Hills police and talked to at least one officer in person, but “it was like pulling teeth getting them to follow up”.

He finally made a formal police report this spring, he said, and said he got little response from the Beverly Hills police until after Dempsey was named on a SeditionHunters website as possible Capitol rioter they referred to as #FlagGaiterCopHater.

After his initial contact with the Los Angeles police last summer, Boyd said, he did not hear back from the department. He followed up most recently this July, also noting that Dempsey had been identified as a potential Capitol rioter.

The Los Angeles police department did not immediately respond to a request for comment on their handling of the assault allegation against Dempsey last summer.

It was not immediately clear if Dempsey had an attorney.

In a video from 6 January, Dempsey is seen delivering a monologue in front of wooden gallows erected by the Capitol, saying that politicians like Nancy Pelosi and Jerry Nadler “don’t need a jail cell. They need to hang from these motherfuckers [pointing to gallows]” and that “the time for peace is over”, according to the criminal complaint.