WASHINGTON - The FBI has determined that the fatal shooting of a U.S. border patrol agent and the wounding of another was a case of friendly fire.

An official said ballistics determined the three agents exchanged gunfire Tuesday in the mistaken belief that each was being fired on by a hostile gunman.  Agent Nicholas Ivie was killed in the gunfight. The shooting occurred about 5 miles from Bisbee, Ariz., as the agents responded to an alarm from a border sensor, indicating movement on the ground.

The area is known as a drug corridor.

A source close to the investigation told The (Phoenix) Republic newspaper  that investigators did not find any evidence of illegal border crossers in the area and described what happened as the "perfect storm." The agents lost radio contact, Ivie got spooked and started to shoot, the source told the newspaper. The other agent shot back and those bullets killed Ivie.

Border Patrol officials held a brief news conference Friday evening to make a statement confirming the friendly fire reports.

Ivie was a 30-year-old father of two. He had been a border patrol agent for four years.

A second agent was shot in the ankle and buttocks and released from the hospital, after undergoing surgery. The third agent wasn't injured.

“I am deeply saddened by the death our fallen colleague," Homeland Security Secretary Janet Napolitano said. "This tragedy reminds us of the risks our men and women confront, the dangers they willingly undertake, while protecting our nation’s borders."