VALHALLA, N.Y. - Investigators from the National Transportation Safety Board have arrived on the site of Tuesday evening's deadly Metro-North accident in Westchester that has left service disrupted along the Harlem Line.

Speaking to reporters in Valhalla, NTSB Board Member Robert Sumwalt says the agency has taken command of the scene and will begin documenting the wreckage immediately.

Six people were killed and 15 others injured around 6:30 p.m. when the eight-car commuter train crashed into a Mercedes SUV.

Although officials stressed that the inquiry was only starting, an early focus was on the driver of the SUV, one of six killed in the crash.

"The question that everybody wants to know is: Why was the vehicle in the crossing?" Sumwalt said.

The train hit the SUV at a two-lane rail crossing on Commerce Street. 

The Mercedes driver and five train passengers were killed.

Investigators say the woman driving the Mercedes got out of her vehicle for a moment after the crossing's safety gates came down around her.

The Mercedes driver got back inside the SUV and was trying to drive forward when she was hit.

The Mercedes was pushed about 1,000 feet down the tracks.

During that time, the NTSB said the third rail penetrated the chassis of the SUV behind and below the driver's seat and came out somewhere below the right rear tire and into the first rail car.

About 400 feet of the third rail started breaking apart in about 80-foot sections and was pushed into mostly the first rail car but also the second car.

The NTSB says the entire interior of the first car was burned down. Initial indications are that the fire was fueled by gasoline from the SUV.

Investigators will be looking into whether the gates were working properly.

The gate arms are supposed to lift automatically if they strike something like a car.

According to Federal Railroad Administration regulations, the lights and arms at a railroad crossing must come down 15 to 20 seconds before the anticipated arrival of a train moving at full speed.

Officials identified the driver of the SUV as Ellen Brody of Scarsdale, N.Y.

One of the victims on the train has been identified as 53-year-old Eric Vandercar. He was a senior managing director for Mesirow Financial in Manhattan.

Walter Liedtke, a curator for the Metropolitan Museum of Art, also died in the crash. The Metropolitan Museum of Art confirmed his death Wednesday evening.

The other victims were identified as 41-year-old Tomar Aditya of Danbury, Connecticut; 36-year-old Robert Dirks of Chappaqua, N.Y.; and 42-year-old Joseph Nadol of Ossining, N.Y.

It's not clear how fast the train was going, though Metro-North officials say the maximum speed would have been 60 mph.

Most of the injured were transported to nearby Westchester County Medical Center.

Four were discharged Tuesday night.

Governor Andrew Cuomo on Tuesday called the scene a truly ugly and brutal site.

"You have seven people who started out today to go about their business and aren't going to be making it home tonight. It's a painful reminder to all of us how precious life is, and sometimes how random it can be," Cuomo said.

Investigators found the train recorders, which should be able to tell them how fast the train was going and whether the brakes were applied.

They will also be looking at the crossing gate mechanisms and the signal system.

The NTSB is also hoping to speak with the conductor within the next day or two.

A source tells NY1 that the train operator, an Air Force veteran who had been on the job for about nine months, was traveling at a speed of less than 60 mph and had put the eight-car train into emergency the instant he saw an obstruction in his path.

While it's still not known how much time the Mercedes driver had to react, a source tells NY1 the train's engineer was trying to stop the train and then tried to evacuate injured riders even after being injured himself.

He is said to be one of the patients being treated in the burn unit.

In a statement, a union representative for the train's crew said, "The engineer and crew performed admirably, if not heroically. It's a tragedy beyond belief."

The MTA itself warned drivers to not pass lowered crossing arms in a 2013 public safety ad that said trains can take up to a mile before they make a complete stop.

A fatality at the same crossing ocurred in October 1984 when a truck driver was struck by a passing train.

In a statement, FRA head Sarah Feinberg said, "Last night’s accident on Metro-North's Harlem line is a tragedy and our thoughts and prayers are with the victims and their families. Federal Railroad Administration investigators are on the scene and we will conduct a thorough investigation to determine the factors that contributed to this accident. Safety must be every railroad's absolute top priority and we will establish what lapses, if any, occurred and order any necessary corrective actions."

Sumwalt made clear that Tuesday's incident was different than other recent Metro-North incidents, including a derailment that led to five deaths outside the Spuyten Duyvil station in the Bronx in December 2013.

"I would be very cautious in trying to draw a nexus between what's happened with Metro-North in the past and what happened yesterday," Sumwalt said.

NTSB investigators say they plan to be on site for about a week.

A full investigation and final report could take up to a year.