PHOENIX (AP) — Friendly fire likely was to blame in a shooting near the Arizona-Mexico line that killed one federal agent and wounded another, the FBI said, noting the investigation was still ongoing in the case that reignited the political debate over border security.

"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 Friday.

Turgal didn't elaborate on its 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. Another agent was wounded but was released from the hospital after surgery. The third agent was uninjured.

Federal investigators used ballistic testing to determine the shootings likely resulted from friendly fire, according to the Cochise County Sheriff's Office, which is assisting the FBI in the probe.

Jeffrey D. Self, commander of Customs and Border Protection's Joint Field Command-Arizona, said that despite the initial findings that the shootings appeared accidental, Ivie still "gave the ultimate sacrifice and died serving his country."

"The fact is the work of the Border Patrol is dangerous," Self said Friday at a news conference in Tucson.

While federal authorities declined to offer details of the shooting, George McCubbin, president of the National Border Patrol Council, said all three agents fired their weapons.

McCubbin told The Arizona Republic that the agents had split up as they investigated the sensor alarm.

"Coming in from different angles, that is more than likely how it ended up happening," he said.

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 didn't respond to telephone messages Friday.

Ivie's funeral is set for Monday in Sierra Vista.

The Border Patrol couldn't immediately comment on the frequency of friendly fire shootings involving its agents. However, 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," Kent Lundgren, chairman of the National Association of Former Border Patrol Officers said, 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.

Ivie's death marked the first fatal shooting of an agent since a 2010 firefight with Mexican bandits that killed U.S. Border Patrol Agent Brian Terry and spawned congressional probes of a botched government gun-smuggling investigation.

The "Fast and Furious" operation 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.