Updated 8:41 PM ET
PHOENIX The FBI said Friday a preliminary investigation has found friendly fire likely was to blame in the shootings of two border agents along the Arizona-Mexico border.
The shootings Tuesday about five miles north of the border near Bisbee left one agent dead and another wounded.
"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 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 in the course of this investigation."
Ivie was shot and 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.
David Klinger, a criminology professor at the University of Missouri at St. Louis and an expert in police shootings, said investigators trying to determine whether friendly fire occurred in a shooting involving law enforcement would compare the ballistics of officers' guns with bullet slugs that were either recovered from or passed through an officer's body.
The officers involved in the case and any known witnesses also would be asked to provide accounts of such a shooting during interviews with investigators. And investigators would try to establish where officers and witnesses were positioned at the time of the shooting, Klinger said.
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 memorial service Monday in Sierra Vista.
The Border Patrol couldn't immediately comment on the frequency of friendly fire shootings at the agency, but such incidents appeared to be extremely rare.
Neither George McCubbin, president of the National Border Patrol Council, nor Kent Lundgren, chairman of the National Association of Former Border Patrol Officers, had ever heard of any friendly fire incidents in the Border Patrol.
"I know of absolutely none in the past, and my past goes back to 1968," Lundgren said, citing the year he joined the Border Patrol. "I'm not saying it never happened. I'm just saying I've never heard of it."
McCubbin has served in the Border Patrol since 1985.
Also Friday, Homeland Security Secretary Janet Napolitano traveled to Arizona to express her condolences to Ivie's family and meet with authorities about the investigation.
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.
On Tuesday after the latest shooting, Sen. Chuck Grassley, R-Iowa, said "there's no way to know at this point how the agent was killed, but because of Operation Fast and Furious, we'll wonder for years if the guns used in any killing along the border were part of an ill-advised gun-walking strategy sanctioned by the federal government." Early investigative work by Grassley, the ranking Republican on the Senate Judiciary Committee, brought Fast and Furious to light in early 2011.
Twenty-six Border Patrol agents have died in the line of duty since 2002.
On Thursday night mourners gathered for a candlelight vigil to remember a U.S. Border Patrol Agent who was killed in a shooting last week.
At a church in this small border town about a hundred people joined to support the family of Ivie, who was remembered as a humble man.
CBS Affiliate KOLD reports Joel Ivie reminisced about his younger brother's short life, and said he spent his final moments doing what he loved.
A priest led the crowd through prayer and talked about respect and appreciation for law enforcement.
The funeral for Ivie is set for Monday in Sierra Vista, Ariz., KPHO reports.