federal grand jury has indicted three North Texas men who officials say are connected to a gun linked to last month’s slaying of ICE Special Agent Jaime Jorge Zapata in Mexico.

Ranferi Osorio, his brother Otilio Osorio and Kelvin Leon Morrison are named in a six-count indictment handed down Wednesday, The Associated Press reported from Dallas.

Each is charged with a single count of conspiracy and four counts of using false statements to a dealer to acquire a firearm. The Osorio brothers are also charged with single counts of possessing a firearm with an obliterated serial number, according to the AP. They remain in federal custody.

Zapata, 32, a native of Brownsville, worked for U.S. Immigration and Customs Enforcement. He was killed last month in Mexico in an attack purportedly by members of the Zetas drug cartel while traveling along Highway 57 in the state of San Luis Potosí. A fellow agent, Victor Avila, was wounded in the attack.

Officials have said at least 15 armed gunmen forced the vehicle Zapata was driving off the highway. Zapata was shot at least three times, officials have said.

The firearms charges aren’t directly related to Zapata’s death, according to the AP.

Mexican authorities have arrested at least two Zeta members in connection with the attack.

Zapata, who grew up in Brownsville and attended the University of Texas at Brownsville and Texas Southmost College, joined ICE in 2006. Before that he worked for the U.S. Border Patrol and had been stationed in Yuma, Ariz. More recently he was stationed in Laredo and had been temporarily working in Mexico City.

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