INCHEON (Yonhap News) ― Five workers were killed when they were hit by a Korea Airport Railroad train in Gyeyang station in the port city of Incheon, authorities said Friday.

Korea Railroad Corp., which operates the train service, said the accident occurred at 0:31 a.m. at the station located 27 kilometers west of Seoul.

The five workers were preparing the railroad tracks at the station so they would not freeze during the winter months. In addition to the five workers killed, another was seriously injured.

The exact cause of the accident is under investigation, but the workers probably entered a part of the track before they were allowed to do so, KORAIL said.

“The workers started their work without approval,” a company official said. “Usually they get permission to start after a train completely passes through the station,” he said.

Police said they are trying to determine why the workers were on the tracks while the airport trains were still in operation. Operation of the line, which connects central Seoul to Incheon International Airport, has normalized after a 15-minute delay.