Federal investigators exploring the wreckage of a fatal Metro-North train crash that killed six people said Wednesday it was not clear yet why an SUV had stopped on the tracks.

But they offered a grim description of an accident that is the deadliest in this railroad’s history, which occurred Tuesday night on a stretch of track in Valhalla, N.Y., about 30 miles north of Grand Central Terminal in Manhattan.

The packed eight-car Metro-North train smashed into a stopped Mercedes SUV and pushed it 1,000 feet down the tracks, causing about 400 feet of the electrified third rail to break apart and stab through the vehicle and into the first rail car, according to Robert Sumwalt, a member of the National Transportation Safety Board.

Pieces of the rail began to break apart into 80-foot sections, most of which remained in the first rail car, though at least one made it into the second rail car, he said. Investigators believe a fire that consumed the first rail car was caused by the SUV’s gasoline.

“The entire interior of the first rail car was burned out,” Sumwalt said during a news conference Wednesday afternoon.

The NTSB dispatched a team of investigators to the scene Wednesday morning, at which point they took over the crash scene and began investigating things like the train’s speed, the mechanical conditions of the rail cars before the crash and the emergency response that followed the crash.

“We intend to find out not only what happened, but we want to find out why it happened,” Sumwalt said earlier Wednesday. “Our sole purpose for being here is to find out what happened so we can offer recommendations to hopefully keep this from happening again.”

While this crash is the latest in a string of accidents that have occurred on Metro-North’s system, drawing criticism from the NTSB, Sumwalt warned that there may not be any relationship between the railroad’s past issues and this crash.

“Here it appears that a vehicle was on the track and the train ran into the vehicle,” he said. “I would be very cautious at trying to draw a nexus between what’s happened with Metro-North in the past and what happened yesterday.”

The train crash killed five people on the train as well as the driver of the vehicle. While officials have not confirmed the names of the victims, the Associated Press identified Ellen Brody, 49, as the driver of the SUV. The AP also named Eric Vandercar, 53, as one of the victims on the train. The Metropolitan Museum of Art released a statement confirming that Walter Liedtke, a curator and scholar of Dutch and Flemish paintings there, was also killed in the crash.

Authorities said it was unclear precisely why the SUV was stopped on the tracks, though one witness said she may have been confused.

Rick Hope, who said he was in the car behind the SUV, told Fox 5 in New York that because of the darkness, “maybe she didn’t know she was in front of the gate.”

Before Tuesday’s crash, a series of accidents since 2013 have brought scrutiny to a railroad that has 83 million riders each year.

There had never been a Metro-North accident with passenger deaths before a train derailed in the Bronx in December 2013. Until Tuesday’s crash, the four people killed in that accident marked the highest death toll in any accident on the railroad.

Authorities had initially said that seven people were killed, but New York Gov. Andrew M. Cuomo (D) said Wednesday morning that the toll had been lowered to six people.

After the Bronx derailment in 2013, the Federal Railroad Administration launched an investigation and issued a scathing report that decried Metro-North’s “deficient safety culture” in the wake of that crash and several other accidents. The report said it was intended to be “an urgent call to action” for the railroad in the wake of these problems. Investigators said they found an ineffective safety department, poor training and an “overemphasis” on punctuality, among other issues.

The same year of the Bronx crash, a derailment and accident in Connecticut injured dozens of people, a worker was struck and killed by a Metro-North train, and a CSX freight train derailed on the Metro-North tracks. Last year, a worker was killed by a Metro-North train in March, and in a separate episode, service was halted for hours because of issues during a repair project.

The NTSB issued a report last November noting that it had launched teams to five significant Metro-North accidents between May 2013 (when the Connecticut derailment happened) and March 2014 (when the worker was struck and killed). This report found that the railroad did not properly use its safety program and that its safety and security department failed to address safety issues.

“Metro-North Railroad did not have an effective system for identifying, monitoring, analyzing, and mitigating safety risks,” it said.

In response, Metro-North’s top official said the railroad was putting safety first and making numerous changes and reforms.

“It is important for Metro-North to acknowledge and learn from what occurred, take corrective action, and move forward,” Joseph J. Giulietti, president of Metro-North, said in a letter posted on the railroad’s Web site. “And that is exactly what Metro-North has done and will continue to do.”

On Wednesday, NTSB investigators were once again in New York exploring the wreckage of a Metro-North crash. The group is being led by rail investigator Michael Hiller, who also oversaw the agency’s investigation into the May 2013 crash in Bridgeport, Conn., in which an eastbound Metro-North train derail and hit a westbound train.

Investigators plan to study the railroad and traffic signals and see what information can be gleaned from the recording devices that were on the train as well as on these signals. The recorders on the train in particular could be key to the investigation, as they show the train’s speed at the time of the crash, which would determine if the brakes were applied before the collision.

The train itself will be moved to a covered storage facility as soon as possible, Sumwalt said.

Cuomo, speaking on Wednesday morning, said that it was too early to know if there were issues with the equipment, the train, the engineer or something else.

“Sometimes there are just accidents,” he said Wednesday morning on “CBS This Morning.” “Sometimes people get themselves in bad situations. So I think it’s too soon to say what’s to blame or who’s to blame.”

Passengers on Tuesday night described a sense of panic as they were evacuated following the collision.

“I just feel guilty quite frankly that I got out and … I don’t think everyone did,” Bruno Maiolo of Ridgefield, Conn., who was on the first train car, told the Journal News of Lohud.

[This post has been updated with additional information throughout the day Wednesday.]