Rod Rothrock, acting sheriff of Cochise County in Arizona, told the Los Angeles Times all three agents were responding separately to an alarm at around 1 a.m. Tuesday from a sensor in a canyon thick with brush. The three were on horseback patrol in Arizona, a few miles from the Mexican border.
Nicholas J. Ivie, who had been a border agent for six years, was shot and killed, while another agent was wounded and the third was not harmed.
Rothrock suggested Ivie may have opened fire on the other agents believing they were displaying an "aggressive posture." He said Ivie was about 20 yards from the other agents.
The FBI has announced a preliminary investigation suggests no outside parties were involved in the shootings.
Two guns were found in the area Tuesday and Wednesday, but investigators say there is no proof they are linked to Ivie's death.