An explosion sparked a massive fire at an oil refinery in Indonesia’s West Java province on Monday, injuring 20 people and prompting the evacuation of nearly 1,000 residents, officials said.

It was not immediately clear what caused the blast.

A fire broke out just after midnight during a thunderstorm, said Ifky Sukarya, spokesman for PT Kilang Pertamina Internasional, a subsidiary of the state-owned energy giant, Pertamina.

“The cause of the fire is still unknown, but at that time it was raining hard accompanied by lightning,” he said.

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“We are focusing on extinguishing the fire,” Sukarya added.

A total of 20 people were injured, including five seriously, and three people were unaccounted for, said Dodi Dwi Endrayadi, the head of the local civil protection agency.

Four students who were walking past the area when the blast happened were injured with serious burns.

Three people were missing after the explosion threw them into a rice field, Dodi said, citing information from witnesses.

A resident died of an apparent heart attack after the blast but was not injured in it, he added.

Nearly 1,000 residents living near the Balongan refinery in Indramayu were evacuated, district chief Nina Agustina said.

She said the blaze had started to subside.

“There was an explosion. It was raining with thunder,”.