A major fire at a lithium battery plant in South Korea killed approximately 23 workers on Monday at a building owned by manufacturer Aricell.

The blaze broke out just after 10:30 a.m. on the second floor of the three-story building, which is owned by Aricell, a lithium primary battery manufacturer, Korea JoongAng Daily reported Tuesday.

“The last missing worker’s body was found earlier in the day, bringing the total death toll to 23,” the outlet said, noting authorities explained the building held 35,000 lithium batteries.

Aricell’s CEO, Park Soon-kwan, offered his condolences and apologies to the families who lost loved ones.

Video footage shows first responders working to put out the fire, while other crews removed what appeared to be bodies from the scene, according to NBC News:

An official said the fire began when battery cells exploded inside the warehouse. However, it remained unknown what triggered the explosion, per the outlet.

According to the Daily report, “Regarding criticism of inadequate safety measures, the company said emergency exit manuals and fire extinguishers were present.”

“Previously, some bereaved families revealed evidence of a prior fire two days before the fatal incident. The company acknowledged this but said it was extinguished immediately and thus not reported to the fire authorities,” the article said.

Approximately 17 Chinese nationals were killed in the fire, the Chinese embassy in South Korea told the Global Times.

The bodies of the victims have been sent for autopsies and officials hope to determine their exact causes of death while also gathering DNA samples to match the victims with relatives, per the Daily.

Aricell manufactures lithium batteries that are used for smart-grid electricity networks and military equipment, according to the Wall Street Journal (WSJ). Aerial video footage shows smoke filling the sky outside of the plant:

The U.S. Coast Guard said an out-of-control lithium-ion battery fire aboard a cargo ship had been put out after it burned for days while the ship was off the coast of Alaska, Breitbart News reported in January.

“Lithium-ion batteries have long been the subject of controversy, with critics questioning their safety,” the article noted.