"There are strong preliminary indications that the death of United States Border Patrol Agent Nicholas J. Ivie and the injury to a second agent was the result of an accidental shooting incident involving only the agents," FBI Special Agent in Charge James L. Turgal Jr. said in a statement.
Turgal didn't elaborate on the agency's conclusions but said the FBI is using "all necessary investigative, forensic and analytical resources" as it investigates the Tuesday shooting about five miles north of the border near Bisbee.
Ivie was killed after he and two other agents responded to an alarm triggered by a sensor aimed at detecting smugglers and others entering the U.S. illegally.
One of the other agents was shot in the ankle and buttocks, but was released from the hospital after surgery. The third agent was uninjured.
The Cochise County Sheriff's Office, which is assisting the FBI in the probe, said federal investigators used ballistic testing to determine the shootings likely were the result of so-called friendly fire among the agents.
Jeffrey D. Self, commander of Customs and Border Protection's Joint Field Command-Arizona, said investigators were making progress and noted the initial findings that the shootings appeared to be accidental didn't diminish the fact that Ivie "gave the ultimate sacrifice and died serving his country."
"The fact is, the work of the Border Patrol is dangerous," Self said during a news conference Friday in Tucson.
While federal authorities declined to offer details of the shooting, George McCubbin, president of the National Border Patrol Council, said the three agents split up as they investigated the sensor alarm, noting they all fired their weapons.
"Coming in from different angles, that is more than likely how it ended up happening," McCubbin told The Arizona Republic of the shootings.
A Mexican law enforcement official said Thursday that federal police had arrested two men who may have been connected to the shootings. The official, who spoke on condition of anonymity because he was not authorized to release the information, said it was unclear if there was strong evidence linking the men to the case.
Mexican authorities on Friday didn't immediately respond to telephone messages from The Associated Press.
After a meeting of border governors Friday in Albuquerque, N.M., Arizona Gov. Jan Brewer stood by the criticism she leveled earlier this week in response to the shootings in which she said a political stalemate and the federal government's failures have left the border unsecured and Border Patrol agents in harm's way.
"It's the federal government's responsibility to secure our border, and they need to do that, and then we can deal with all the other issues that have come about because our border hasn't been secured," said Brewer, who plans to attend Ivie's funeral Monday in Sierra Vista.
The Border Patrol couldn't immediately comment on the frequency of friendly fire shootings involving its agents. But such incidents appeared to be extremely rare, if they've ever occurred at all.
"I know of absolutely none in the past, and my past goes back to 1968," said Kent Lundgren, chairman of the National Association of Former Border Patrol Officers, citing the year he joined the agency. "I'm not saying it never happened. I'm just saying I've never heard of it."
Also Friday, Homeland Security Secretary Janet Napolitano traveled to Arizona to express her condolences to Ivie's family and meet with authorities. The family did not return calls from The Associated Press on Friday.
Ivie's death marked the first fatal shooting of an agent since a deadly 2010 firefight with Mexican bandits that killed U.S. Border Patrol Agent Brian Terry in December 2010 and spawned congressional probes of a botched government gun-smuggling investigation.
Terry's shooting was later linked to that "Fast and Furious" operation, which allowed people suspected of illegally buying guns for others to walk away from gun shops with weapons, rather than be arrested.
Authorities intended to track the guns into Mexico. Two rifles found at the scene of Terry's shooting were bought by a member of the gun-smuggling ring being investigated. Critics of the operation say any shooting along the border now will raise the specter that those illegal weapons are still being used.
Twenty-six Border Patrol agents have died in the line of duty since 2002.
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