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A former U.S. Air Force staff sergeant and alleged member of the “boogaloo” extremist movement pleaded guilty Friday in the fatal shooting of a federal security officer in the San Francisco Bay Area amid large 2020 protests against police brutality.

Steven Carrillo, 33, changed his plea to guilty to a federal murder charge in the killing of David Patrick Underwood and to the attempted murder of Underwood’s colleague after federal prosecutors last month agreed not to seek the death penalty.

The men were shot on May 29, 2020, while they stood in front of a federal building in Oakland as hundreds marched on the streets.

Dressed in an orange jumpsuit in federal court, Carrillo read from a plea agreement, admitting to posting messages on Facebook a day before the shooting asking anyone if they were “down to boog” and saying he was ready to act and not just talk. He also admitted firing 19 rounds from a homemade AR-15 rifle from the back of a white van being driven by a man he connected with online.

“I aligned myself with the anti-government movement and wanted to carry out violent acts against federal law enforcement officers in particular,” Carrillo said.

Prosecutors on Jan. 31 said they would not seek the death penalty, but U.S. District Judge Yvonne Gonzalez Rogers said Friday she is not convinced that she will accept the plea agreement between prosecutors and defense lawyers calling for a 41-year prison sentence.

“I cannot accept a plea unless there was a sufficient independent factual basis for the plea,” she said.

Carillo is scheduled to be sentenced on June 3 but Gonzalez Rogers warned that she could reject the plea agreement if she doesn’t feel prosecutors and defense lawyers do enough to justify the sentence.

And if she does, Carillo would go to trial and his admissions in court could be used against him, Gonzalez Rogers said.

Prosecutors have said Carrillo, of Santa Cruz, had ties to the “boogaloo” movement —a concept embraced by a loose network of gun enthusiasts and militia-style extremists. The group started in alt-right culture on the internet with the belief that there is an impending U.S. civil war, according to experts.

Authorities accused Carrillo of fatally shooting Underwood after spraying a guard shack he was in with bullets.

Underwood’s sister, Angela Underwood Jacobs, called Carillo a “domestic terrorist” during Friday’s hearing.

“Cowards like you fear true bravery,” she said.

Prosecutors said Robert Alvin Justus Jr., of Millbrae, drove the van. He faces federal charges of murder and attempted murder in the case.

The pair is accused of driving to Oakland and taking advantage of the distraction afforded by people marching through the city’s downtown to protest George Floyd’s killing by a police officer in Minneapolis.

Carrillo was arrested a week after the shooting in Oakland after he allegedly ambushed sheriff’s deputies in Santa Cruz County who were responding to a report of a van containing firearms and bomb-making materials. Sgt. Damon Gutzwiller, 38, was killed and several other law enforcement officials were wounded, according to authorities and court records.

Prosecutors in Santa Cruz charged Carrillo with a slew of felonies, including murder and attempted murder in connection to that killing.

Carrillo pleaded not guilty to Gutzwiller’s killing.