PHOENIX – The FBI said Friday a preliminary investigation has found friendly fire likely was to blame in the shootings of two border agents along the Arizona-Mexico border, shaking up the probe into an incident that re-ignited the political debate over security on the border.

The shootings Tuesday about five miles north of the border near Bisbee left one agent dead and another wounded.

“While it is important to emphasize that the FBI’s investigation is actively continuing, there are strong preliminary indications that the death of United States Border Patrol Agent Nicholas J. Ivie and the injury to a second agent (were) the result of an accidental shooting incident involving only the agents,” FBI Special Agent in Charge James L. Turgal Jr. said.

Ivie was shot and killed after he and two other agents responded to an alarm triggered by a sensor aimed at detecting smugglers and others entering the United States illegally. Another agent was shot in the ankle and buttocks, but was released from the hospital after surgery.

Arizona Gov. Jan Brewer stood by the criticism she leveled earlier this week in which she said a political stalemate and the federal government’s failures have left the border unsecured and agents in harm’s way.