PHOENIX (AP) — A new report into a shooting that left a U.S. Border Patrol agent dead says three agents responding to an alarm were apparently in radio contact as they approached from opposite directions before opening fire on each other in the Arizona desert.

A sheriff’s report released Friday says it was a clear night and the agents were on patrol separately when the call came in at about 1:30 a.m. Oct. 2 that an underground sensor aimed at detecting smugglers and illegal immigrants had been tripped.

Agent Nicholas Ivie, 30, approached on foot from the north. The two other agents walked in from the south when Ivie apparently opened fire, eliciting a deadly barrage of return fire from his colleagues.

Ivie was killed. Another agent was wounded. The third wasn’t injured.

Questions had swirled as to whether the agents were in radio contact with each other in the rugged, hilly terrain where signals can be spotty. A communication breakdown could have led to the confusion and ensuing shootout.

However, according to the preliminary report by the Cochise County sheriff’s office, which is investigating the case along with the FBI, the uninjured agent later told authorities "they were in radio communication with Agent Ivie."

"At one point she observed (Ivie) signaling them with his flashlight," according to the report.

The agent, whose name hasn’t been released, told investigators "as they were walking up the trail she heard yelling and then observed muzzle flashes in front of her and heard gunfire."

"She drew her weapon and took cover," the report states.

The agent also said she thought she saw several people moving through the darkness and whispering after the shooting.

The FBI declined to comment on the report, citing its own ongoing investigation.

Authorities have said the agents were about 20 yards from each other when the gunfire erupted. Acting Cochise County Sheriff Rod Rothrock has characterized the shooting as accidental, adding he didn’t expect any criminal charges to be filed.

The FBI has called it a case of friendly fire but has declined to comment on any other aspects of the investigation.

One sheriff’s deputy noted in the report that authorities initially believed it was a "possible ambush."

In the immediate aftermath of the shooting, FBI agents flew in on a Black Hawk helicopter, and heavily armed law enforcement officers secured the area, flooding it with personnel looking for potential assailants. Two people were detained in Mexico but were later released after they were found to have no connection to the shooting.

The report from the sheriff’s office details the painstaking efforts to document the scene, collecting bullet casings, tracking unidentified footprints and following a blood trail. It mentioned six spent rifle casings clustered in the area, and that the only items not found holstered in Ivie’s belt were his flashlight and pistol.

"His radio was still on/operating," the report stated.

While the report sheds some light on the shooting, key details remained unanswered, such as just why Ivie opened fire contrary to training that dictates agents should only discharge their weapons once they have identified a suspect as hostile.

"None of them should have started firing unless they were returning fire from a smuggler or whatever the perpetrator might have been," said retired Border Patrol Agent Jim Dorcy.

While it’s clear something went terribly wrong, no one is publicly questioning Ivie’s competency or his training. He’s been described by fellow agents as an experienced law enforcement officer with six years at the Border Patrol, the kind of agent others looked up to.

U.S. Rep. Silvestre Reyes, D-Texas, a former Border Patrol sector chief in El Paso, said agents are trained for every potential scenario, but he noted "things often go contrary to plan," especially in a nighttime situation "in an area you know you might encounter bad guys."

"Your heart is racing. You’re trying to identify who is where. Ideally, you’re in communication with other agents, but it’s not a finite situation where everything always works out," Reyes said. "That said, the job is dangerous enough when you’re up against the bad guys, so you want to make sure you avoid any kind confrontation among what is commonly referred to as ‘friendlies,’ but we just don’t know what happened out there yet."