An explosion in an underground electrical vault in Downtown Long Beach left five people hospitalized with burns this morning, a fire official said.

Officers heard the explosion around 4:10 a.m. near Golden Shore and West Seaside Way, according to Lt. Tony Lopez of the Long Beach Police Department. The blast appeared to have happened in an underground vault just below the Golden Shore bridge over Shoreline Drive.

A crew of private contractors hired by Southern California Edison was working in the vault when the explosion happened and Long Beach firefighters were called to the scene, Lopez said.

Firefighters rescued two men who were still in the vault and five people were transported to hospitals to treat burns that did not appear to be life-threatening, authorities said.

“The extent of their injuries is not known at this time,” Edison said in a statement.

Four of the patients were taken by paramedics, and the fifth was taken in a basic-life-support ambulance, meaning that person’s injuries were less severe, Long Beach Fire Capt. Matt Dobberpuhl said.

Tuesday afternoon, Edison said three of the five workers had been discharged from the hospital.

It wasn’t immediately clear what caused the explosion.

“SCE has initiated an investigation,” the utility said. “The contract crew was working on a scheduled maintenance outage at the time of the incident.”

About 73 customers ended up without power because of the accident, according to Edison. Later Tuesday, that number dwindled to a dozen.