(CNN) -- The FBI is looking into the possibility that the fatal shooting of Border Patrol Agent Nicholas Ivie and the wounding of another agent may have been the result of friendly fire, a law enforcement official said Friday.
The official said investigators at the shooting scene, which is amid rugged terrain, have found no shell casings except those believed to have been fired by Border Patrol agents. The official stressed other evidence might yet be found.
Investigators are waiting for the results of ballistics tests, said the official, who was not authorized to speak on the record about the ongoing investigation.
Meanwhile, the head of the Department of Homeland Security was traveling to Arizona Friday, a day after Mexican authorities questioned two men in connection with the shooting near the U.S.-Mexico border.
The Mexican army handed the two over to local authorities in Sonora and they are being detained near the American border, the Mexican attorney general's office said Thursday. The two were in possession of drugs and guns when they were detained, said a source in the same office.
U.S. authorities said Ivie and the other agent came under fire after responding to a sensor that had gone off near Naco, Arizona.
Ivie, a 30-year-old Provo, Utah, native who joined the Border Patrol in January 2008, is survived by his wife and two young children.
Homeland Security Secretary Janet Napolitano and other federal officials will meet with Ivie's family "to express their condolences," said department spokesman Matt Chandler.
Ivie is the 14th agent killed in the line of duty since 2008, including three this year.
He was killed near a border station recently named for Brian Terry, whose 2010 death led to the public disclosure of the botched Fast and Furious gun-smuggling sting, according to Sen. Chuck Grassley of Iowa.
The agent who was wounded has not been identified. After the shooting, he was airlifted to a hospital with non-life-threatening injuries.
He was released from the hospital Wednesday, said Jeremy Copeland, an agent with the Tucson Sector of U.S. Border Patrol.
In addition to Mexican authorities, the FBI is conducting a joint investigation with the Cochise County Sheriff's Office.
Lanny Breuer, assistant attorney general for the Criminal Division of the Justice Department, said earlier this week that investigators were at the scene.
"Every time that a law enforcement person is either killed or shot or injured in the line of duty, we have to take a moment and think of our families and think of the heroes involved," Breuer said.
CNN's Rafael Romo and Eduardo Aragon contributed to this report.