A former Air Force staff sergeant who allegedly killed a federal officer during a drive-by shooting that left another officer injured outside a San Francisco-area federal building during protests against police brutality pleaded guilty Friday. 

Steven Carrillo, 33, initially pleaded not guilty but changed his mind after federal prosecutors agreed not to seek the death penalty. 

Authorities said he fatally shot Federal Protective Services Officer David Patrick Underwood on May 29, 2020 and attempted to kill Underwood’s colleague. He belonged to the "boogaloo" extremist anti-government movement, they said.

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Steven Carrillo is accused of killing two law enforcement officers in California last year was part of a right-wing militia. He pleaded guilty to one of the killings Friday.  

Steven Carrillo is accused of killing two law enforcement officers in California last year was part of a right-wing militia. He pleaded guilty to one of the killings Friday.   (AP/Santa Cruz Sheriff's Office)

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 read from the plea agreement in court.

He allegedly sprayed a shack Underwood was in with bullets. During Friday's hearing, Underwood's sister, Angela Underwood Jacobs, called Carrillo a "domestic terrorist."

"Cowards like you fear true bravery," she said.

U.S. District Judge Yvonne Gonzalez Rogers warned that she may not accept the plea agreement. Prosecutors and defense lawyers have called for her to impose a 41-year prison sentence. 

Carrillo was allegedly arrested in possession of the gun – a privately made firearm with no manufacturer’s markings.

Carrillo was allegedly arrested in possession of the gun – a privately made firearm with no manufacturer’s markings. (Justice Department )

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

Robert Alvin Justus Jr., who prosecutors allege drove the van during the killing, faces federal murder and attempted murder charges. 

In addition to the Underwood slaying, local prosecutors have charged Carrillo with murder and attempted murder, among other felonies, in connection with the death of Santa Cruz County Sheriff's Sgt. Damon Gutzwiller.

Carrillo was arrested in Oakland a week after shooting Underwood after he allegedly ambushed San Cruz County sheriff's deputies responding to reports of a van containing firearms and bomb-making materials. Several law enforcement officials were injured.  

Robert Alvin Justus Jr.

Robert Alvin Justus Jr. (Justice Department )

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He has pleaded not guilty to Gutzwiller's killing.

The Associated Press contributed to this report.