The FBI said there are strong indications in their preliminary investigations that point to a shooting incident involving only agents.
Authorities say three agents were responding to an automatic alarm near the Mexican border. They reportedly split up and then began firing.
According to investigators, ballistic tests showed that the death of agent Nicholas Ivie was the result of accidental shooting between the agents.
Another agent was wounded, and released from a hospital after surgery.
A spokesman for the Ivie family said how the agent died changes little.
"Quite honestly, the circumstances that surround exactly what happened will do nothing to bring Nick back," Kevin Goates, Sierra Vista stake president for the Church of Jesus Christ of Latter-day Saints, told The Arizona Republic. "Those are just details.
"The fact is he is gone, and that is what the family is focusing on and their time together and their time for healing," Goates said.
Ivie is survived by his wife, Christy, and two young daughters, plus his parents and siblings.
The Associated Press contributed to this report.