A probe began Wednesday into what caused a crowded Metro car to leave the tracks during the afternoon rush a day earlier, forcing passengers to evacuate through a tunnel that remained closed as investigators searched for answers.

The National Transportation Safety Board worked at the scene of the Blue Line derailment through the day, looking for clues into what caused Metro’s 7000-series train to slip off the tracks in Northern Virginia. While none of the 187 passengers were injured, the safety board’s involvement turned up scrutiny on Metro’s safety protocols and emergency response, signaling that the federal agency saw a safety incident that warranted a deeper look.

NTSB spokeswoman Jennifer Gabris said the agency chose to investigate because it “could advance a known safety issue.” She added: “In this case, based on previous WMATA [Washington Metropolitan Area Transit Authority] accidents that also involved tunnels and derailments, NTSB decided to investigate.”

In an internal memo sent to transit agency staff 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.

“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.

Train service on the Blue Line between the Rosslyn and Arlington Cemetery stations remained suspended Wednesday during the investigation, and it wasn’t known whether service would be restored Thursday. Free shuttle buses are operating between the three affected stations: Rosslyn, Arlington Cemetery and the Pentagon.

The train derailed about 5 p.m. Tuesday between Rosslyn and Arlington Cemetery. Arlington County Fire Department officials said one car slipped off the tracks. Firefighters and Metro Transit police officers evacuated passengers, one of whom was sent to a hospital as a precaution for anxiety issues, fire officials said.

The train had left Rosslyn, then 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 left the tracks in Silver Spring. No one was injured in that incident, which investigators said occurred after the train operator ran a red signal.

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

Radio dispatches recorded by openmhz.com during the derailment indicate the train operator reported a holding brake being stuck before Train No. 407 left the Rosslyn station and went off the tracks.

“Four, zero, seven leaving Rosslyn,” a train operator said, “got a stuck holding brake.”

In later dispatches, a Metro worker says it looked as if cars 7200 and 7201 had pulled apart, while another dispatch reported the rear of 7200 being on the ground.

“From our angle, it looks like 7200 … wheels came off the track,” the employee said. “We’ll have to have a better look as we offload.”

Onboard the train, passengers were complaining of smoke entering rail cars.

“Customer’s punched me up [and] said car 7200′s filling up with smoke,” a Metro worker reported over the radio to Metro’s Rail Operation Control Center.

Metro officials declined to comment on the dispatches or investigation, referring questions to the NTSB. Metro spokesman Ian Jannetta said transit agency board members will receive an update during their scheduled Thursday meeting.

In his memo, Wiedefeld said Metro 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.”

Investigators from the Washington Metrorail Safety Commission also worked at the scene Wednesday. The agency declined to comment on the investigation.

“Our people are still out there,” commission spokesman Max Smith said. “We’re gathering information and that’s part of the NTSB investigation.”

Russell G. Quimby, a former NTSB investigator for more than two decades, said the safety board’s decision to take the lead on the investigation could grow out of its belief that an investigation could lead to instructive safety practices or protocols beneficial to all rail transit.

“The safety board tends to cherry-pick [cases] because they’ve got so few people,” he said. “This is somewhat presumptuous because you don’t know what you’re going to find when you get into an investigation, but they pick accidents that they think are going to have some kind of a safety benefit.”

Before 2016, Metro’s lengthy record of safety incidents prompted frequent investigations from the federal agency, leading to a rare Federal Transit Administration takeover of Metrorail oversight between late 2015 and 2019.

Quimby said the agency also could have chosen to investigate the derailment because the transit system may not have acted upon recommendations the NTSB made in the past.

“[Metro] has had some issues in the past, and I think they might be looking at this to do kind of a follow-up on past recommendations that they’ve done,” he said.

NTSB records show that Metro has at least seven open or closed recommendations from the federal agency dating to the 1980s that are marked as having “unacceptable” responses.

They include establishing a program to identify operators who are at high risk for obstructive sleep apnea or other sleep disorders and requiring them to be evaluated and treated. Two more call on Metro to make sure all train operators are trained and regularly tested on how to shut down rail car ventilation — a factor that contributed to the death of a passenger during a smoke event in 2015.

Unresolved NTSB recommendations over the past three decades have been discussed at Washington Metrorail Safety Commission meetings as recently as 2019. Metro declined to comment Wednesday.