The two sets of agents approached an area where a sensor had been activated early Tuesday from different directions and encountered each other in an area of heavy brush, National Border Patrol Council president George McCubbin said.
Agent Nicholas Ivie apparently opened fire first and wounded one of the other agents but was killed in the return fire.
"I don't know what it was he saw or heard that triggered this whole event," McCubbin said. "Unfortunately it resulted in his death and another agent injured."
Acting Cochise County Sheriff Rod Rothrock confirmed the scenario but would not say if Ivie was the first to shoot, saying that was up to the federal agencies involved.
The new details add to a FBI statement Friday that the shooting appeared to be a friendly fire incident that involved no one but the agents.
Sensors are set up in different areas along the U.S.-Mexico border to detect smugglers or illegal immigrants, with Border Patrol agents responding when they're set off. The shooting occurred in a rugged hilly area about five miles north of the border near Bisbee, Ariz., an area known for illegal trafficking.
McCubbin and Rothrock both said the two sets of agents knew the others were heading to the area on foot but apparently didn't know they were so close. McCubbin said he'd been briefed by the agency, while Rothrock's agency has been involved with the investigation.
"It was dark, very, very rugged terrain, and what they could see of each other was further obscured by the fact that there was brush and cacti and stuff like that between them," Rothrock said. "I have no doubt that these agents were in as heightened a state of alert as you can get due to the proximity to the border and the history of trafficking in that area."
Rothrock said that when the agents spotted each other in the dark, "they apparently took defensive postures, which was probably interpreted as aggressive postures. Like readying your weapons, for example."
Ivie, 30, died at the scene, and one of the other agents was wounded but has since been released from the hospital.
In the immediate aftermath of the shooting, the Border Patrol and other federal and local agencies flooded the area with personnel looking for who they believed were assailants who had attacked the agents.
"That was the initial reports from the beginning," McCubbin said. "That was the reason for the saturation manhunt there. They even had permission to fly into Mexico. They were seeking people out. What this was based on, we'd have to assume it was based on the initial statements given by the agents on the scene."
Two people suspected of being involved in the shooting were arrested by the Mexican government but were apparently not involved at all.
"They had a couple of people in custody but other than being in the area, there was no evidence putting them there at the scene," McCubbin said. "They could have been guides, they could have been scouts, and those type of folks typically go back and forth all the time anyway."
Rothrock said he believed the agents' actions were "appropriate and in accordance with their training had they, in fact, been engaging people involved in illegal activities. Unfortunately, they weren't engaging people involved in illegal activities, they were engaging each other."
Rothrock said he doubted any legal action will be taken against the surviving agents.
Ivie's funeral is set for Monday morning Sierra Vista, Ariz..
"It's happened and it's a horrible tragedy for the agents involved and their families and the agency," McCubbin said. "We can come up with some reasons as to how this happened but that doesn't fix anything. All we can do is send prayers to the families and all the agents involved that somehow they can find some peace with this someday."
Ivie's death marked the first fatal shooting of an agent since a deadly 2010 firefight with Mexican gunmen that killed U.S. Border Patrol Agent Brian Terry in December 2010 and spawned congressional probes of a botched government gun-smuggling investigation.