An explosion at a law enforcement training facility in Los Angeles has killed three people with the county sheriff’s department in the largest loss of life for the agency since 1857, the sheriff said on Friday morning.

The three deputies who were killed were members of the department’s arson explosives detail said Robert Luna, the sheriff, at a press conference. Authorities were still working to notify relatives of the deceased, he said, and details on the circumstances around the explosion were limited.

“There’s a lot more that we don’t know than what we do know, but our intent is to look at this from the very beginning and figure out what is it exactly that caused this tragic event,” Luna said.

Investigators do not believe there is any threat to the community.

The incident took place shortly after 7.30am on Friday when an explosion occurred at the department’s Special Enforcement Bureau, in the arson explosives detail facility in East Los Angeles. The three people killed had worked for the agency, collectively, for 74 years, Luna said.

“The individuals who work our arson explosives detail, they have years of training,” Luna said, adding that they frequently deal with dangerous calls. “They are fantastic experts, and unfortunately, I lost three of them today.”

The bomb squad of the Los Angeles police department responded to the scene to render the devices safe, the sheriff said, a task that took hours. The FBI and ATF were on scene to assist with the investigation.

The US attorney general, Pam Bondi, told reporters that federal agents were at the scene of Friday’s explosion and were working to learn more. She said her information came from US attorney Bill Essayli’s office, whose jurisdiction includes Los Angeles.

The office of California’s governor, Gavin Newsom, said he had been briefed on the blast. The Department of Homeland Security secretary, Kristi Noem, said the agency was working with state and local law enforcement.

Aerial footage from KABC-TV showed the explosion happened in a parking lot filled with sheriff patrol cars and box trucks.

Hilda Solis, a Los Angeles county supervisor, said in a statement she was “deeply saddened by the tragic incident”, and in contact with the sheriff as the investigation develops.

Reuters and the Associated Press contributed reporting.