Three Los Angeles County Sheriff’s Department deputies were killed today in an explosion at a training facility in East Los Angeles, a tragedy Sheriff Robert Luna called his agency’s deadliest day since 1857.

The blast was reported at 7:25 a.m. Friday in the 1000 block of North Eastern Avenue, southwest of the interchange of the 10 and the 710 freeways, according to the Sheriff’s Information Bureau.

“There is definitely a black cloud hanging over the Los Angeles County Sheriff’s Department and our county family,” Luna said during a midday news conference near the facility.

The sheriff assured the community the blast was an “isolated incident” and there was no lingering threat to the community. While the blast occurred around 7:25 a.m., Luna said the site where the blast occurred was not rendered safe by investigators until late morning.

“We don’t know the cause of the explosion, however, the specific area has been closed off,” Luna said.

The sheriff would not confirm reports that the blast involved unexploded ordnance that had been seized at an unknown location on Thursday.

“There’s a lot more we don’t know that we do know,” Luna said, adding, “We want to make sure we know what happened. We don’t want a repeat of this.”

The explosion occurred at the Biscailuz Regional Training Center, which serves the sheriff’s Special Enforcement Bureau’s Arson/Explosives Detail. Luna said the deputies who died were all part of the Arson/Explosives detail and were long-time veterans of the department. One had 19 years of experience, another 22 years’ experience and the third had 33 years with the department.

Police and politicians from across the state and country expressed their condolences, with Long Beach Police Chief Wally Hebeish releasing a statement that said his department joins the sheriff’s department “in mourning the loss of three of their, who served with dedication and honor.”