An explosion in a factory complex in Ulsan left three people injured, authorities said Wednesday.

An explosion occurred around 8:25 a.m. in an oil storage tank at the petrochemical firm Kolon Industries’ Ulsan plant. Three subcontracted workers who had climbed the tank to connect a pipeline to the container fell from a height of 1.5 meters in the explosion. One of the workers sustained first-degree burns.

The petroleum resin plant was reportedly conducting a regular maintenance operation, and the investigative authorities believe the explosion resulted from a sudden increase in the pressure of the remaining vapor inside the tank.

The local police and fire agency are investigating the exact cause of the explosion.