According to the Korea Herald:
Doola Shipping, which operates the ship, said the explosion seems to have taken place while gas was being drained from the oil tank. "The vessel usually transports diesel, but this time it carried gasoline. We are now examining whether it had any relation to the explosion," a company official said.
The explosion was not caused by any external shock, Coast Guard officials said, adding it suddenly occurred when some crew members were cleaning the oil tank on the deck. They will request a forensic test for the case, the officials said.