A fire believed to be caused by arson broke out on a Seoul Subway Line Five train Saturday morning, sending multiple passengers to the hospital.

Authorities say the fire started at around 8:40 a.m. inside a passenger car as the train was heading toward Mapo Station from Yeouinaru Station.

Some 400 passengers evacuated through a tunnel. Among them, 21 people were sent to the hospital for smoke inhalation or injuries.

The train's engineer and some of the passengers reportedly used fire extinguishers to help contain the flames. The fire was fully contained by 10:24 a.m., with 166 firefighters, 60 police officers, and 68 pieces of equipment dispatched to the scene.

Police arrested a man, assumed to be in his 60s, on suspicion of arson. Witnesses say the man boarded the train carrying an oil container, and set some clothing on fire using a torch lighter.