A two-alarm fire on the rooftop of Metro’s new headquarters being built near L’Enfant Plaza caused a plume of black smoke that was visible over downtown Washington late Thursday morning.

       Firefighters worked quickly to extinguish the flames on top of the 10-story building in the 300 block of 7th Street SW, four blocks south of the Mall. Officials briefly closed one entrance to the L’Enfant Plaza Metro station, although rail service wasn’t affected.

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       Authorities said the fire, reported about 10:35 a.m., was confined to construction materials. The cause is under investigation, and Metro said it appears to be accidental. The fire department said two firefighters suffered minor injuries and were taken to a hospital for treatment.

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       Officials said 20 firetrucks and other apparatus, along with more than 100 firefighters, responded to the scene.

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       Vito Maggiolo, a spokesman for the D.C. fire department, said the building had interior water pipes, called standpipes, that firefighters used to connect hoses on upper floors. Maggiolo said firefighters also carried 50-foot-long hoses to the roof, supplied by a street-level water line.

       Construction workers in the building got out safely, Maggiolo said.

       Metro has long planned to move out of its nearly 50-year-old headquarters near Capital One Arena.

       In October 2018, the transit agency bought a vacant building at 300 Seventh Street SW for $53.1 million. It planned to expand the eight-floor building to 11 floors and remodel the structure into a rectangular high-rise enclosed by giant glass panes. Metro said it would occupy the first eight floors and lease out the top three.

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       In February 2020, Metro announced it had reached a deal to lease its existing downtown headquarters to a development group that would remake the building into a modern commercial office space that could house more than 1,500 employees.

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       Metro is expected to vacate the current building around December 2022. The agency said on Twitter it doesn’t anticipate Thursday’s fire will cause “any significant impacts” to those plans.

       In late May 2020, a two-alarm fire damaged the agency’s existing headquarters. No one was injured in the blaze, which prompted the relocation of some rail control operators. Most employees, however, had not been working in the building because of the pandemic.