CHICAGO (CBS) — The pedestrian tunnel linking the Red and Blue lines at Jackson Boulevard was closed for several hours Monday evening.

Firefighters were investigating a potentially hazardous material in the tunnel, which runs underground between State and Dearborn streets.

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Pedestrians were redirected to other entrances to the Chicago Transit Authority train stations.

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As of late Monday, officials still did not know what the suspicious substance was.

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No one was injured.