(CNN) The FBI is investigating two brothers as "persons of interest" in a possible attack two weeks ago that left one Border Patrol agent dead and another seriously injured near Van Horn, Texas.

The bureau, following up on a tip from an informant, is pursuing a theory that the brothers, undocumented immigrants allegedly involved in smuggling people, drugs or both, might have assaulted the two agents, according to an unsealed search warrant filed in federal court in Albuquerque, New Mexico, Monday.

The warrant, first reported by TV station KRQE in Albuquerque, New Mexico, sought access to a 2004 Pontiac Grand Am driven by the brothers.

No charges had been filed as of Wednesday evening relating to any possible assault, and the FBI hasn't named either man as a suspect, so CNN is not naming them at this time.

Customs and Border Protection officials referred questions to the FBI, which declined to comment, citing the ongoing investigation.

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