An FBI agent believes U.S. Border Patrol Agent Rogelio Martinez was murdered, according to a sworn affidavit filed in a federal court this week.

Martinez died Nov. 19, after suffering head injuries the night before while on duty in Texas.

Border Patrol Agent Stephen Garland was found injured alongside Martinez, in a culvert off Interstate 10, in Van Horn, Texas. He has been unable to recall what happened to them, authorities said.

In the affidavit, the FBI agent requests a search warrant for a 2004 Pontiac Grand Am that he believes “will contain trace evidence relating to drug trafficking, the murder of Agent Martinez and the assault of Agent Garland.”

Last week Culberson County Sheriff Oscar Carrillo told the Dallas Morning News he believed a tractor-trailer may have inadvertently struck the agents, Fox News reported.

In the affidavit, the FBI investigator said an informant told a Customs and Border Protection agent that two brothers who were recently smuggled into the United States spoke about assaulting the agents.

Further investigation found that those two brothers are Antonio and Jesus Munoz, authorities said. But upon questioning, both brothers denied any role in the attack on the agents, according to the affidavit.