WASHINGTON, Jan.12-- One person died and dozens hospitalized after smoke filled a subway tunnel and a major Metro station in Washington, D.C. Monday afternoon, officials said.

A spokesperson for the Washington Metropolitan Area Transit Authority (WMATA) confirmed the incident, which occured during afternoon rush-hour in the U.S. capital city, partly paralyzing the nation's second-busiest public transport system.

So far, the source of the smoke has not been determined, said a WMATA statement.

A total of 84 passengers were taken to hospital and more than 200 people were evaluated after the subway tunnel and the L'Enfant Plaza subway station, one of the city's busiest stations near several government buildings, were engulfed with thick fog, said fire officials.

Washington's Metro system serves a population of 5 million within a 1,500-square-mile area, according to WMATA data. The metro rails alone serve more than 700,000 customers a day.