A submarine explosion that occurred at a Navy repair facility left three military personnel killed and one injured, Defense Ministry said Tuesday.

The explosion, which happened around 8:30 a.m. in Jinhae, South Gyeongsang Province, is presumed to have been caused by the gas accumulated within the vessel, a ministry official explained.

“The submarine had been placed in the repair facility for a regular inspection from May 2 to Sept. 2, and was about to be moved for repairs today,” a military official said. He added that the military is conducting an investigation on specifically how the accident had occured.

The officials in question were not of the Navy but at a separate division within the Defense Ministry. Earlier in the day, local media reported that they were members of the Republic of Korea Defense Intelligence Command.

The vessel has been in service for the past 30 years.

Military said it has alerted the bereaved families of the deceased and is working on follow-up measures for the accident.

By Yoon Min-sik(minsikyoon@heraldcorp.com)