UPDATE: The NTSB has launched an investigation into the fatal crash. Click here for more.

Six people were killed when a Metro-North train packed with commuters from New York City hit a car on the railroad tracks in Westchester at the height of evening rush hour, sparking a fiery crash that's also injured at least 12 people, officials say.

The train out of Grand Central Terminal was going northbound on the Harlem line when it struck a Jeep Cherokee at the Commerce Street crossing in Valhalla at about 6:30 p.m., officials said.

The Jeep was stopped on the tracks when the railroad crossing gates came down on top of it, according to MTA spokesman Aaron Donovan. The driver got out to look at the back of the car, then got back in and drove forward when the train struck the Jeep, pushing it about 10 train-car lengths up the track.

The Jeep driver and five passengers on the train were killed in the crash, Gov. Andrew Cuomo said in a news conference Tuesday night. The crash is the deadliest in the history of the Metro-North Railroad. Officials initally said six passengers had died, but MTA officials downgraded the death toll Wednesday morning.

"This is truly an ugly, brutal sight," Cuomo said. "The third rail of the track came up from the explosion and went right through the car, so it is truly a devastatingly ugly situation to see."

The rail did not hit any passengers, officials said.

The National Transportation Safety Board said Tuesday it is launching a team to the crash site to investigate. Cuomo and MTA Chairman Tom Prendergast said an event recorder on the train will help piece together the circumstances surrounding the crash.

Chopper 4, first over the scene, showed a massive emergency response as the front train cars smoldered for hours, sending thick plumes of smoke into the air.

A photo posted by Justin (@bizzz23) on Feb 3, 2015 at 3:50pm PST

Six hundred and fifty people were on board train No. 659, which departed Grand Central at 5:45 p.m. The train made one stop at 125th Street in Harlem before it proceeded express toward Chappaqua, the MTA said.

Passengers' accounts of the crash varied according to where they were sitting along the train. Those in back reported only feeling only small "jolts" or "jerks" upon impact, while those sitting toward the front heard a loud explosion and saw smoke quickly filling their cars.

Jamie Wallace, who was sitting in the back of the second car, recalled how everyone escaped the train after the crash.

A photo posted by Justin (@bizzz23) on Feb 3, 2015 at 4:06pm PST

"We started to hear people further up toward the head of the second car start to panic a bit, and they were calling for a fire extinguisher," he said.

But as passengers rushed to help, "we could not get the head car doors open for some reason, it was jammed," he said. "We then were trying to break the glass to no avail."

A photo posted by malkam dior (@discoverdior) on Feb 3, 2015 at 6:29pm PST

"A number of us started smelling fumes from the car, the fuel, and we said, 'you know what, we need to get out.' The fire was starting to spread back toward the second car, and the second car finally did ignite," he said.

Another passenger named Fred was on the fourth train from the front. He hurt his fingers when he smashed his hand through emergency glass during the evacuation.

"The thing that precipitated people really starting to freak out and break the glass and open the door was there was a loud 'bam,' explosion-type thing, and once we jumped off the side, there was another explosion to a lesser degree," he said.

Neil Rader of Katonah was sitting in the middle-back of the train when he felt a "small jolt."

"It felt not even like a short stop, and then the train just completely stopped," he said.

He said passengers in his car also had to evacuate by breaking glass on the doors to get out. He said he saw 50 to 60 ambulances at the scene as he walked to a nearby gym, which was acting as a holding area for the escaped train riders.

"I've never seen anything quite like it," said Rader.

Stacey Eisner, an NBCUniversal News Group employee, was sitting in one of the rear two cars of the train, and said she felt the train "jerk" at some point. The conductor walked through the train to explain what had happened, and passengers were calm at first, but tension began to build when they learned the train had hit a car, she said.

About 10 to 15 minutes after the train "jerk," Eisner's train car was evacuated, with ladders used to get people out. People were taken either to a nearby rock-climbing gym called The Cliffs or allowed to walk to the Hawthorne Funeral Home, she said.

Ryan Cottrell, assistant director at The Cliffs, told NBC News that the passengers who were brought there appeared to be shaken up but generally OK. Staffers who saw the incident from the front door went to the scene to help bring passengers into the gym, where they were providing shelter and warmth until MTA buses arrive to transport commuters to Pleasantville, Cottrell said.

A few injured people were transported from the gym to the hospital, said Cottrell.

The train engineer was treated for his injuries at a hospital, but was not considered one of the casualties, Prendergast said.

The Taconic State Parkway, which runs parallel to the Metro-North tracks in the area, was closed in both directions in the town of Mount Pleasant as police, EMS and firefighters responded.

The Metro-North Harlem line will remain suspended Wednesday between Pleasantville and North White Plains. There will be limited bus/train service for Upper Harlem line customers beginning with morning rush hour service on Wednesday until further notice.

Normal train service remains between Grand Central and North White Plains, the MTA said.

Harlem line tickets will be cross-honored on the Hudson and New Haven lines. Up to 300 parking spaces at the Westchester County Center and up to 50 parking spaces at the North White Plains Station will be available on a first-come, first-served basis.

The collision comes a little more than a year after new Metro-North President Joseph Giuletti took over, with a vow to make safety the top priority.

Multiple derailments in 2013 and 2014 -- including one in December 2013 that killed four people when a fatigued engineer fell asleep at the controls -- had prompted a federal review in which investigators concluded that Metro-North sacrificed safety in 2013 to accommodate an obsession with on-time performance.

The MTA says it has made dozens of recommended changes, but big-ticket items like automated train control could still be months or even years away. It appears too early to tell whether safety controls could have prevented Tuesday's collision.

U.S. Sen. Charles Schumer, a vocal advocate for improved safety on the Metro-North, said in a statement he's spoken to Prendergast, who said that "a full and thorough investigation has already begun."

"At this early stage, it is premature to point any fingers of blame, but there are many important questions that must be answered in the coming days," Schumer said.

Metro-North has established a family assistance center at the Mount Pleasant Town Hall at 1 Town Hall Plaza in Valhalla and a phone hotline at 1-800-METRO-INFO (800-638-7646).