(U.S. Customs and Border Protection)

Authorities are looking closely at the possibility that a friendly fire accidental shooting is at the heart of the incident that killed Border Patrol Agent Nicholas Ivie and wounded a second agent, sources familiar with the investigation told ABC News.

A third agent was unharmed in the incident.

The FBI Phoenix field division would only say in a statement that, "Due to the sensitive and ongoing nature of this investigation, the FBI is unable to provide any further details at this time."

The scenario that is being examined is whether the fatal shooting happened in the "fog of war" following some type of encounter.

Additional forensics are being done on ballistics and additional interviews are being conducted to determine if the matter can be resolved in the coming days.