FBI officials say they are investigating the death of a border patrol agent and injuring of another in West Texas as a "potential assault," but they wouldn't rule out that they were injured in some other way.

Special Agent in Charge Emmerson Buie Jr. said during a news conference at the FBI's El Paso office Tuesday that the bureau is offering a reward of $25,000 for information that might lead to a resolution of the case.

The officials wouldn't say why they believe the agents might have been attacked.

Agent Rogelio Martinez died Sunday after suffering extensive injuries to his head and body. His partner, whose name hasn't been released, was seriously injured.

A U.S. official with knowledge of the investigation told the Associated Press on Monday that investigators believe Martinez may have fallen into a culvert near Van Horn. The official said Martinez's partner, who radioed for help, has no memory of what happened.