The explosives that detonated and killed three L.A. County Sheriff’s Department deputies may have been found in a storage unit at an apartment complex in Santa Monica, a law enforcement source tells local affiliate KTLA.

The source then reported that authorities initially thought the explosives, military-style grenades, were inert before they cut one open the next day as part of a training exercise.

The blast occurred last Friday morning at the Biscailuz Regional Training Center in East Los Angeles.  

The three deputies, Detective Joshua Kelley-Eklund, Detective Victor Lemus and Detective William Osborn, were killed while moving “undetonated ordnance,” sources told KTLA on Friday. All three were assigned to the LASD Special Enforcement Bureau’s Arson Explosives Detail.

New reports emerging Monday morning indicate that the explosives may have been connected to an operation that took place last Thursday – the day before the blast – in Santa Monica. Deputies, including the three who perished, were called to an apartment building on Bay Street to assist Santa Monica Police Department officers with an investigation into explosive devices found in a storage unit near the building.

A source told KTLA that the explosives found in the apartment’s storage unit on Thursday may be what killed the deputies on Friday. It was later reported by the same source that the explosives, two military-style grenades, were X-rayed at the scene and deemed inert on Thursday.

The grenades, according to the source, had been stolen by a member of the U.S. military during their training. The three deputies loaded them into their unit and took them back to East Los Angeles.

Per the law enforcement source, on the following day, the deputies cut one of the grenades in half as a training exercise in order to examine what was inside of it thinking it would not explode; however, that is when the device detonated.

“According to the source, it was a tragic mistake or accident that caused the men to lose their lives,” KTLA 5’s Eric Spillman reported.

Spillman reached out to the Sheriff’s Department for official confirmation; however, they were not able to verify what the source said.

A resident of the Santa Monica apartment complex said that a woman calling 911 after she found the explosives in her storage unit, which likely belonged to the previous tenant, who he confirmed was once a member of the military.  

“The man who lived here a number of years ago was ex-military,” the resident explained to KTLA. “He had some firearms, as well as, apparently, grenades.” 

“We all have storage units, and the woman who currently lives in the apartment he used to live in found these grenades in her storage unit – they were hidden in some way, or it seems like they were forgotten,” he elaborated.

The explosive devices recovered on Thursday were then brought back to the East L.A. facility by authorities, where the deputies were believed to be moving them when they detonated on Friday.

Bomb squad technicians were seen at the apartment complex later on Friday investigating the area and evacuating residents while searching for any more explosive material. It was not immediately clear whether any was found.

KTLA is still awaiting official confirmation from the Sheriff’s Department on whether the explosives found in Santa Monica were the ones that detonated and killed the deputies.