EL PASO, Texas -- The FBI questioned two brothers from New Mexico in the death of a border patrol agent in Texas, according to federal court documents that have been sealed. Jeanette Harper, a spokeswoman for the FBI's El Paso office, said Thursday that the investigation is ongoing into the death of agent Rogelio Martinez and serious injuries sustained by his partner.

FBI officials said previously that they were investigating the incident as a "potential assault" but they have not ruled out that the injuries could have been caused by an accident.

The search warrant and affidavit were filed Monday in New Mexico federal court and have since been sealed. In the affidavit, the investigators asking for a warrant to search a car belonging to the brothers, referred to them as "likely perpetrators of the assault."

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But, court documents show the search warrant was executed on Dec. 3, a day before the FBI doubled its reward seeking information in the case.

Neither brother has been charged in 36-year-old Martinez's death. One of the brothers was being held on an immigration-related offense as of Thursday.

Martinez's partner radioed for help while the two were investigating a report of activity on Nov. 18 outside of Van Horn, Texas, about 110 miles southeast of El Paso and about 30 miles from the border with Mexico. The two were found with serious injuries including head trauma and broken bones.

Martinez died at a hospital a few hours later. His partner has recovered, but investigators say he doesn't remember what happened.

The court documents said a woman who had ridden in the car with the brothers submitted to the tip to investigators.

Harper said the FBI has received numerous tips, but declined to elaborate on the brothers or other tips they may be investigating.

"The FBI has not pressed charges against any individual, and we are still continuing with the investigation," she said.