The Ginza subway line was halted for about 20 minutes during the morning rush hour Friday due to smoke from a disaster drill that flowed into the subway system.

The cause of the smoke was not immediately known, leading the subway operator to halt all the services on the line.

But it soon found out that the smoke came from warning flares used in the drill in Tokyo’s Ginza district and that it flowed into the subway tunnel between Ginza and Shimbashi stations through air vents.

Service resumed after the smoke was found to have posed no danger. The incident affected about 14,000 people, it said.

A female commuter who works at a department store in the Ginza district said: “I’ve never heard of a subway stopping for a disaster drill.”

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