PHOENIX (AP) — The FBI says a preliminary investigation has found friendly fire is likely to blame in the shootings of two border agents along the Arizona-Mexico border.

The shootings occurred Tuesday in a rugged area about five miles north of the border near Bisbee, Ariz.

Agent Nicholas Ivie and two others responded to an alarm triggered by a sensor aimed at detecting smugglers and others entering the U.S. illegally. Ivie was fatally shot. The wounded agent was struck in the ankle and buttocks and released from the hospital after surgery. The third agent wasn't injured.

Ivie's death marked the first fatal shooting of an agent since a deadly firefight with Mexican bandits that killed U.S. Border Patrol Agent Brian Terry in 2010 and spawned congressional probes of a botched government gun-smuggling investigation.

Twenty-six border agents have died in the line of duty since 2002.