The border agent who was killed Sunday while patrolling in Texas near the U.S.-Mexico border was beaten to death by assailants who used rocks to crush his skull, an official with knowledge of the situation told The Daily Caller News Foundation.

Rogelio Martinez and his partner were working in the Big Bend sector when they reported they had been injured and were in need of assistance, according to Customs and Border Protection (CBP).

Responding agents transported both agents to a local hospital, where Martinez passed away from his injuries. His partner, whose name has not been released by CBP, remains hospitalized in serious condition.

In a statement released Sunday, CBP confirmed Martinez’s death but did not provide details about the circumstances leading to the incident. Shortly after CBP’s confirmation, Breitbart Texas reported that representatives of the National Border Patrol Council (NBPC) were told by local border agents that illegal immigrants attacked Martinez and his partner.

“What we know is that Border Patrol Agent Rogelio Martinez appears to have been ambushed by a group of illegal aliens whom he was tracking,” Brandon Judd, president of the NBPC, told Breitbart Texas. “Our agents’ reports from the ground say that he was struck in the head multiple times with a rock or rocks.”

That account was independently confirmed by a CBP official who spoke on the condition of anonymity because they were not authorized to discuss the ongoing investigation.

“They were beaten with rocks,” the official told TheDCNF. “As in rock in hand and smashing it into their heads. They were beaten.”

CBP did not provide further details about Martinez’s death when reached for comment. The FBI’s El Paso office is leading the investigation, a CBP spokesperson said.

Martinez, 36, was from El Paso and entered duty with the Border Patrol in August 2013.

Department of Homeland Security Acting Secretary Elaine Duke called Martinez’s death a “tragic event” and said the agency is “fully supporting the ongoing investigation” of the incident.

“On behalf of the quarter of a million frontline officers and agents of DHS, my thoughts and prayers go out to the family and friends of Agent Martinez and to the agent who is in serious condition,” she said in a statement Sunday.

Follow Will on Twitter

Send tips to will@dailycallernewsfoundation.org.

Content created by The Daily Caller News Foundation is available without charge to any eligible news publisher that can provide a large audience. For licensing opportunities of our original content, please contact licensing@dailycallernewsfoundation.org.