Story highlights Secretary Napolitano travels to Arizona to express her condolences, meet with officials

"There are strong preliminary indications" that the shooting was accidental, says the FBI

Source: Only casings believed to have been fired by agents have been found

One Border Patrol agent killed and another wounded in shooting this week in Arizona

This week's fatal shooting of a U.S. Border Patrol agent and the wounding of another in Arizona was likely the result of friendly fire, the FBI said late Friday.

"While it is important to emphasize that the FBI's investigation is actively continuing, there are strong preliminary indications that the death of United States Border Patrol Agent Nicholas J. Ivie, 30, and the injury to a second agent was the result of an accidental shooting incident involving only the agents," James Turgal, special agent in charge of the FBI's Phoenix division, said in a statement.

"At the appropriate time further information will be provided, but while the investigation continues it would be inappropriate to comment any further at this time," he said.

Earlier, a law enforcement official said investigators at the scene had not found shell casings except those believed to have been fired by the Border Patrol agents.

Investigators are awaiting results of ballistics tests, said the official, who was not authorized to speak for attribution about the investigation.

"We have much to learn and conclude from this incident, and I ask for the public's patience and understanding during this difficult time," Jeff Self, commander of U.S. Customs and Border Protection's Arizona field joint command, told reporters late Friday.

He remembered Ivie for his character, kindness and loyalty.

"Agent Ivie gave the ultimate sacrifice and died serving his country. ... He died in the line of duty and will be honored as such for his final act of service," said Self.

Meanwhile, Homeland Security Secretary Janet Napolitano traveled to Arizona on Friday to express her condolences to the fallen agent's family and to meet with local officials.

"This tragedy reminds us of the risks our men and women confront, the dangers they willingly undertake, while protecting our nation's borders," she said. "Together, we stand in solidarity with their families and friends, and pray for the continued safety of all who serve our country."

Napolitano was in Arizona one 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 were 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.

Ivie, a Provo, Utah, native who joined the Border Patrol in January 2008, is survived by his wife and two young children.

Ivie was the 14th agent killed in the line of duty since 2008, including three this year.

The agent who was wounded has not been identified. After the shooting, he was airlifted to a hospital with non-life-threatening injuries, and later released.