Smoke was detected on a Tokyo subway line on Thursday, forcing operators to temporarily stop the trains, but there were no injuries, the subway operator and police said.
A conductor on the Marunouchi subway line saw smoke coming from a track in the mid-afternoon, but it was not initially clear whether he saw any fire, said Jiro Umegaki of Tokyo Metro Co. Ltd.
Police and firefighters rushed to the scene but there was no fire, and investigators found no sign of a possible crime, said Nobutaka Furusawa, deputy chief of the nearby Nakano Police Department, the AP reports.
All traffic on the line was halted until officials confirmed that smoke from a construction site just above the spot had escaped into the subway tunnel through a ventilation duct, another subway official Rumiko Meguro said. The subway line resumed full service about an hour later, just before the evening rush hour, she added.
The smoke caused delays for about 22,000 passengers.