LAREDO, Texas (AP) — An off-duty U.S. border agent has been arrested in connection with the fatal shooting of a firefighter in the Texas border city of Laredo, authorities said.

U.S. Customs and Border Protection Officer Ernesto Gillen, 43, was charged with murder in Wednesday's death of of Guadalupe David De Luna. Neither Gillen nor De Luna were on duty at the time, and investigators determined that De Luna was shot after a domestic altercation, the Laredo Morning Times reported.

“It was a domestic incident involving a romantic relationship,” Laredo police spokesman Jose Baeza told the San Antonio Express-News.

Baeza said the two men were fighting, but wouldn't provide more details.

Laredo Fire Department Chief Steve E. Landin said that De Luna, 42, was a 21-year veteran with the department.

“I want to offer the De Luna family the department’s deepest condolences on the loss of one of our firefighters,” said Fire Chief Steve E. Landin. “Our organization prays for him and his family."

Officers found De Luna with a gunshot wound while responding to a woman's report of a shooting at an apartment complex, according to police.

Gillen was taken to the Webb County Jail, where he was being held without bond. If convicted, he faces a punishment of up to 99 years in prison or a life sentence.

In a statement, U.S. Customs and Border Protection said Gillen had 12 years of service with the agency and that he is currently in a “non-duty” status. The agency added that it’s cooperating with investigators.