Three deputies were killed Friday in an explosion at a law enforcement training facility in Los Angeles, Sheriff Robert Luna said.

"This is, unfortunately, the largest loss of life for us as the LA County Sheriff's Department, since 1857," Luna added.

The explosion occurred in the parking lot of the Special Enforcement Bureau at the Biscailuz Center Academy Training Facility, an equivalent of what is commonly known as a SWAT team.

Authorities were investigating the cause of the explosion, but it could take several weeks.

The Los Angeles Times quoted sources who said a bomb squad was moving explosives when the blast occurred.

FBI, arson investigators and LAPD bomb squad deployed 

US Attorney General Pam Bondi said federal agents had been deployed to the site of what she called a "horrific incident."

Homicide detectives were on the scene, as well as agents from the Federal Bureau of Investigation, the Los Angeles Police Department bomb squad and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

Sheriff Luna says that three members assigned to the department's "arson explosive detail" died in the explosion.

"They have years of training," he said. "They are fantastic experts and, unfortunately, I lost three of them today."

It is not clear what the deputies were doing at the time of the explosion.

A possible training accident was an early focus of the investigation, according to a law enforcement official who spoke to The Associated Press on condition of anonymity because they were not authorized to discuss the matter publicly.

Edited by: Rana Taha