The National Transportation Safety Board began its investigation Wednesday into a Metro train derailment that forced passengers off a crowded train a day earlier near Arlington Cemetery.

       In an internal memo to transit agency staff early Wednesday, Metro General Manager Paul J. Wiedefeld instructed employees to fully cooperate with federal investigators. He said it’s not clear why the train left the tracks.

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       “Every employee who is asked for information or involved in the investigation is expected to fully cooperate with the NTSB and be governed by their process,” Wiedefeld wrote in the memo obtained by The Washington Post.

       Investigators from Metro and the Washington Metrorail Safety Commission will work with the NTSB, he said. The commission is an independent agency created by Congress to oversee Metrorail safety.

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       Train service on the Blue Line between Rosslyn and Arlington Cemetery remained suspended Wednesday during the investigation. Free shuttle buses are running between the three affected stations: Rosslyn, Arlington Cemetery and the Pentagon.

       Metro train derails on the Blue Line

       The Blue Line train derailed Tuesday afternoon between the Rosslyn and Arlington Cemetery stations in Northern Virginia. Arlington County Fire Department officials said one car slipped off the tracks. Firefighters and Metro Transit police officers evacuated 187 passengers from the train, one of which was sent to a hospital as a precaution for anxiety issues, fire officials said.

       The train had left Rosslyn just before 5 p.m., when it slowed and sputtered to a stop, according to a passenger onboard. Smoke seeped into the train, and some passengers were told to move to a different car while they waited nearly two hours to be evacuated.

       The derailment is the first of a Metro train carrying passengers since July 7, 2020, when a Red Line train carrying 32 riders slipped off the tracks in Silver Spring, Md. No one was injured in that incident, which investigators said occurred after the train operator ran a red signal.

       Red Line train derails after operator overruns red signal, Metro safety commission says

       In April, a rail maintenance machine used for overnight track work derailed outside the Farragut West station, but no injuries were reported.

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       In the latest derailment, Wiedefeld said the transit agency will support investigators and cooperate “in a fully transparent manner.”

       “We share a sense of urgency to understand why the derailment occurred,” he wrote. “However, until the investigation is complete, we will not comment further nor speculate on the potential cause(s) of the derailment itself. The NTSB will identify the probable cause(s) and we will take appropriate actions to prevent a similar accident in the future.”

       This story will be updated.