Law enforcement officials Tuesday announced murder charges against two men, and the case is now officially connected to the killing of a Santa Cruz County Sheriff's deputy eight days later.
I-TEAM: Air Force sergeant arrested on suspicion for killing of deputy in Santa Cruz County.
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According to the complaint, an active duty Air Force police officer left Travis Air Force Base that day and went to Oakland with an accomplice to kill cops.
Bullet holes are still visible at the Oakland Federal Building guard post, where 53-year-old security officer Pat Underwood lost his life and his partner was seriously injured May 29.
In that same complex Tuesday, federal law enforcement officials announced charges in the attack.
U.S. Attorney David Anderson said, "Pat Underwood was murdered because he wore a uniform. He wore that uniform to signify his authority to protect the federal courthouse where we are gathered here today."
The complaint contains photographs of a white van that parked at the intersection across from the guard post. It says 30-year-old Robert Justus of Millbrae was the driver.
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"A man later identified as Justus got out of the van and walked around the area for approximately 10 minutes, in a way, suggesting that he was conducting surveillance."
The complaint says, after 17 minutes, the van pulled into the intersection and that 32-year-old Steven Carrillo, an active duty Air Force police officer from Travis Air Force Base, slid the back door open and fired what's called a "ghost gun," parts purchased separately and assembled from scratch.
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Pat Gorman, ATF Special Agent in Charge, said, "The firearm used in these crimes was a privately made firearm, with no markings or serial number. This firearm is a machine gun, which had a silencer attached to its barrel."
Federal agents also confirmed Tuesday the same van and gun was part of the ambush that killed Santa Cruz County Sheriff's Deputy Damon Gutzwiller on June 6.
The complaint says Carrillo's ballistic vest he had that day bore a patch with a symbol of the "Boogaloo" movement.
Anderson explained, "The Boogaloo term is used by extremists to reference a violent uprising, or impending civil war in the United States.
The complaint contains a photo, alleging that Carrillo wrote "Boog" in blood on the hood of a car before his arrest, and some of the movement's sayings including "stop the duopoly" and "I became unreasonable."
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The complaint says Carrillo and Justus met on Facebook and that, on the morning of the attack, Carrillo commented, "Go to the riots and support our own cause. Show them the real targets. Use their anger to fuel our fire. Think outside the box. We have mobs of angry people to use to our advantage."
The FBI says the men used the protests in Oakland that night as cover for their killing.
Agents are studying video from SKY7 that may show the van shortly before the attack.
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FBI Special Agent in Charge Jack Bennett said, "There is no evidence that these men had any intention to join the demonstration in Oakland, as some of the media have asked. They came to Oakland to kill cops."
RELATED: 38-year-old deputy killed, 2 officers injured after ambush in Santa Cruz County, sheriff says
Federal officials also said they had Justus under surveillance and that they watched as his parents drove him to the FBI in San Francisco to turn himself in last Thursday.
Neighbors in Millbrae confirmed for ABC7 News that Justus lived in an apartment along El Camino Real.
WATCH: Neighbors react to suspect Robert Justus' alleged involvement in killing of Oakland federal officer
Neighbors react to suspect Robert Justus' alleged involvement in killing of Oakland federal officer
Off-camera, they said they noticed nothing alarming about his behavior. Instead, they described him as someone who often said hello and goodbye in passing.
Justus' apartment sat above several Millbrae businesses. Employees admitted they weren't familiar with Justus, although they confirmed the FBI recently stopped by to ask for any surveillance footage of the area. Workers explained they didn't have any footage to provide.
Online, photos taken from Justus' Facebook page showed he publicly displayed the symbol of the "Boogaloo" movement.
Read the full complaint against Carrillo here and the complaint against Justus here.
Take a look at more stories and videos by the ABC7 News I-Team.