The president of the Border Patrol agents union said agent Nicholas Ivie may have been the one who opened fire first on two fellow agents, apparently mistaking them for smugglers.
National Border Patrol Council President George McCubbin said it appears Ivie shot at two of his fellow agents first and was killed when they returned fire.
The shooting happened early Tuesday morning in a well-known smuggling area, thick with heavy brush.
McCubbin said two sets of agents, including Ivie, responded to a tripped sensor.
He said they arrived from different directions and apparently Ivie fired the first shot, mistaking his fellow agents for armed smugglers.
McCubbin said it's unclear exactly what prompted him to start shooting.
Ivie was killed, a second agent was wounded and the third wasn't hurt.
Friday the FBI confirmed they are investigating the shooting as a case of friendly fire.
"I explained to agent Ivie's family that if the investigation ultimately reaches that conclusion, it changes none of the following facts that agent Ivie gave the ultimate sacrifice and died serving his country," said Border Patrol Commander Jeffrey Self.
Meanwhile, friends and family gathered in Sierra Vista Sunday for agent Ivie's visitation. He left behind his wife Christy and two young daughters.
"We know he's with loved ones that have passed on before we know well be able to see him again and that's definitely what's carrying us through," said Chris Ivie, Nicholas' brother.