A Metro train partially slipped off the tracks on the Blue Line near Arlington Cemetery on Tuesday evening, causing delays and suspending Blue Line service between Pentagon and Foggy Bottom. The National Transportation Safety Board will take the lead in investigating the incident, according to an email sent to Metro staff on Wednesday morning.

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

There were no injuries reported initially on Tuesday, according to WMATA spokesperson Ian Jannetta. Shuttle buses replaced trains between the stations.

It was not immediately clear what caused the train to slip off the tracks. A representative for the Washington Metrorail Safety Commission, an independent body that audits Metro’s safety performance and investigates serious safety incidents, was on scene at the derailment. The commission will work alongside NTSB and Metro as the investigation into what happened progresses.

Some reports indicated that passengers were evacuated to the Arlington Cemetery station by walking along the train tracks. Metro did not confirm that account or the number of affected passengers.

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The Metro Rail Operations Control Center was alerted of a problem with a train in-between Arlington Cemetery and Rosslyn stations at 4:53 p.m., and personnel confirmed that the train had partially derailed shortly after. The train was a 7000-series, the transit agency’s newest model.

Metro’s fiscal year 2021 goal is to have four derailments in a year or fewer.

This story has been updated to include more information on the investigation into the incident, and to reflect that some service has been suspended, rather than full station closures.