A US Air Force sergeant with links to the far-right Boogaloo Bois movement has been charged with the murder of a federal security officer in California, the FBI says.
Steven Carrillo is accused of killing David Patrick Underwood outside the courthouse in Oakland during Black Lives Matter protests last month.
He was already charged with the murder of another officer eight days later.
Damon Gutzwiller was killed in an ambush near Santa Cruz on 6 June.
Mr Carrillo, who is stationed at Travis air force base, was arrested during a subsequent confrontation.
He appears to have used his own blood to write various phrases on the bonnet of a car he stole, the FBI said, including "boog" and "stop the duopoly".
"Boogaloo" is a term used by extremists to reference a violent uprising or impending civil war in the US, the FBI said.
Adherents of the loose grouping known as Boogaloo Bois, which some liken to a militia, are anti-government and often carry assault weapons.
The Oakland victim, Mr Underwood, was killed in a drive-by shooting from a white van overnight on 29 May. There were large protests taking place in nearby streets over the death in police custody of George Floyd.
A colleague was also shot in the attack, and the accused is charged with his attempted murder.
A second man, Robert Alvin Justus, has confessed to driving the van, the FBI said, and has been charged with aiding and abetting the alleged murder.
Who are the Boogaloo Bois?
By Shayan Sardarizadeh, BBC Monitoring
A loose online movement with no leaders, Boogaloo Bois grew out of a firearms-themed bulletin board on the 4chan platform.
The group's name is a reference to a 1984 film, Breakin' 2: Electric Boogaloo, which members use to refer to an armed conflict with authorities and law enforcement, something akin to a civil war.
While it is hard to ascribe a specific ideology to Boogaloo Bois, followers sign up to two fundamental issues: a desire for an armed overthrow of the government, and an unwavering commitment to guns.
The movement has considerably grown in size in the last year, helped by Facebook groups and pages with tens of thousands of members and followers. Alternative terms like "Big Igloo", "Boog" and "Big Luau" are also frequently used.
Armed members dressed in Hawaiian shirts and military fatigues claim they are attending Black Lives Matter rallies to "protect the protesters from police".
But they are increasingly capitalising on the protests to attack the authorities. Three Boogaloo members were charged with terrorism offences in Nevada this month for attempts to "spark violence".
Facebook has limited the reach of Boogaloo groups and pages in recent days. But many are still active on the platform.
In a comment on Facebook, Mr Carillo said the Black Lives Matter protests in downtown Oakland were a good opportunity. "Use their anger to fuel our fire. Think outside the box. We have mobs of angry people to use to our advantage," he allegedly wrote.
Announcing the charges at the same Oakland federal building where Mr Underwood worked, US Attorney David Anderson said many people would miss hearing the sound of his voice and laughter.
"Pat Underwood wore his uniform because it signified his authority to protect the courthouse where we are gathered here today," the attorney said. "This courthouse exists to administer justice, to uphold the rule of law, and to protect the freedoms that we all cherish."
A second deputy was injured in the attack in Santa Cruz county in which Sgt Gutzwiller lost his life.
The sheriff's office is holding a memorial event for Sgt Gutzwiller on Wednesday which will be broadcast online.