Federal investigators have arrived at the scene of a deadly crash in Westchester between a Metro-North commuter train and a sport utility vehicle to find out "what happened and why it happened" with the goal to prevent a similar accident in the future, a National Transportation Safety Board member said at a news conference Wednesday.

Six people were killed and 15 people were injured Tuesday evening when the rush-hour train struck the SUV in Valhalla, the Metropolitan Transportation Authority said Wednesday, revising the number of people who died. Officials initially said seven people had died.

Five people on the train were killed along with the driver of the SUV, the MTA said in a news release.

Meanwhile, doctors at Westchester Medical Center said 4 patients had already been discharged, while 9 remained: 1 in critical, 1 in serious, 4 in fair and 2 in good. Injuries included lacerations, contusions, open fractures, smoke inhalations, crush injuries, dislocations and flame burn injuries, said Dr. Ivan Miller, Emergency Department Director.

Anyone seeking information on loved ones can head to a family assistance center at the Office of the Westchester County Medical Examiner in Valhalla or call 1-800-METRO-INFO.

During a briefing about the accident on Wednesday morning, the NTSB said it was just getting started with its investigation.

"We don't have a lot of hard facts," NTSB board member Robert Sumwalt said.

"We've got several busy days ahead of us," he said, with the goal being to "find out what happened and why it happened . . . to keep this from happening again."

Sumwalt said the NTSB would likely be at the scene for about a week, "collecting perishable evidence, evidence that can go away with the passage of time."

The NTSB will use a 3-D laser scanning device to scan the wreckage, and it has asked for high-definition aerial footage of the site. The agency will also be interviewing witnesses.

Sumwalt said investigators hope to interview the train's operators within 24 to 48 hours, understanding that they've been through emotional and maybe physical trauma.

"We want to make sure they're in a position that they can comfortably speak to investigators -- but that's a high priority," he said.

Recorders on the trains, signals and crossing gates should also help investigators determine what happened and whether there was a change in the train's speed, a whistle blown, and when the gates and signals were activated.

Sumwalt asked witnesses who have information they would like to share with the agency to email witness@ntsb.gov.

Sumwalt said he anticipates the train will be moved to storage later Wednesday afternoon.

"By the time we've completed this investigation we will know everything we need to know," Sumwalt said.

MTA spokesman Aaron Donovan said he didn't expect the names of those who died to be released Wednesday.

"Notifying next of kin will take some time," Donovan said.

The conductor and engineer were both injured, Donovan said, but he was unsure the extent of their injuries or whether they were hospitalized.

"A minor miracle from last night is there's one less body," Westchester County Executive Rob Astorino said Wednesday morning at a news conference across the street from the scene of the crash.

Astorino said that except for one of the people who died, "all of them were burned beyond recognition, so we're going to need dental records to try to identify the bodies."

With the charred, twisted wreckage of the train about 100 feet behind him, Astorino said about 400 feet of third rail will need to be replaced, and the train will have to remain on site until the NTSB releases it.

"For those who were there last night, it was a horrific scene. Personally this is my town, this is where I commuted to when I was a commuter, and my stop is the next stop, in Hawthorne. To think of what these commuters went through," Astorino said. "That train had so many flames in it, it was so engulfed, the inside of that first car is melted and charred with the third rail going through it."

Gov. Andrew M. Cuomo on Tuesday night described the crash scene as a "truly ugly and brutal sight."

The governor described a scene of twisted metal, after a hurling mass of fire rocketed down the tracks and engulfed both the SUV and the train's first car. He said after touring the wreckage that it was "amazing" more hadn't died in the fiery crash.

The Metro-North Train 659, typically packed with more than 600 passengers at rush hour, left Grand Central Terminal at about 5:44 p.m, MTA officials said. At about 6:30 p.m. and nearly 26 miles north of Grand Central, the train hit a stalled Jeep Cherokee, officials said. Witnesses described seeing a giant ball of flames immediately after the crash.

According to the MTA, the train struck the SUV at the Commerce Street railroad grade crossing in Valhalla, adjacent to the Kensico Cemetery. It was unknown how fast the train was traveling at impact, but officials said they can reach speeds of 60 mph.

"It basically created a fireball," said Derrick Gilliam, of Hawthorne, who witnessed the crash.

Gilliam said he was driving toward Commerce Street when the train struck the vehicle.

"It was just this fireball moving along the tracks, pushing the vehicle. The whole first car of the train was just a big ball of fire. . . . It was a direct hit," he said. "Just a tremendous, tremendous impact."

Shauna Marcus, 34, who had just left a rock-climbing gym near the crash scene, was headed toward her parked car when, she said, she saw a black Jeep Cherokee get struck by the lowering railroad bars at the crossing.

"The woman got out . . . to look at the SUV and see what was wrong . . . like, to assess the damage. But she was right on the tracks."

Marcus said she got in her own car and started to drive off, then heard a massive crash.

"I turned and saw the fire and I knew what happened," she said. "She just stopped or got stalled on the tracks at the worse possible moment."

"It's just the most unlucky thing that can happen," Marcus said.

Westchester County Executive Rob Astorino said, "I've never seen anything like it. The whole front car of the train is melted inside from the flames, and the third rail actually came up from underneath the train, through the whole car.

"I'm amazed that anyone got out alive."

MTA chief Tom Prendergast said the maximum speed was 60 mph in that area and typically such a train would carry 655 people.

Dozens of law enforcement and fire officials were on scene in the snow-covered area late Tuesday. Helicopters buzzed overhead.

About 400 customers were taken to the rock-climbing gym for shelter.

The train was going to the North White Plains yard.

An NTSB spokesman said the agency was sending a "go" team to investigate the crash.

Kary Williams, 38, general manager of The Cliffs Climbing and Fitness, said dozens of passengers were brought there, some treated for injuries, others to find a way home.

"People are in shock," he said. "The paramedics and all the rescue crews have done an amazing job. The biggest thing is everyone is in a little bit of disbelief."

Several people at Westchester Medical Center, in Valhalla, were huddled together in the emergency waiting room late Tuesday night, frantically asking hospital officials for information on missing loved ones.

Mount Pleasant Supervisor Carl Fulgenzi said he expected to meet with the NTSB, chief of police, "and other officials to give us the details" before midnight.

Sen. Charles Schumer released a statement saying, "Our hearts go out to those lost, we pray for those injured and our hats are tipped to the brave first responders who came to the scene of this tragic crash so quickly. I have spoken to [MTA chief] Tom Prendergast, who has assured me 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.

Normally, MTA spokesman Aaron Donovan said, "flashing red lights and gates . . . come down to prevent cars from going across the tracks, so they will investigate what happened in this case, whether the cars drove around or whether there was a malfunction of some kind."

Tuesday's accident comes as Metro-North is still reeling from a spate of safety-related incidents in recent years, including a half-dozen accidents in less than 12 months that claimed six lives.

Chief among those was the Dec. 1, 2013, Bronx derailment that killed four passengers and injured 61 when a Metro-North engineer reportedly fell asleep at the controls and sped through a sharp curve.

The accident spurred a federal investigation that found that Metro-North -- once considered the standard for American commuter railroads -- was routinely putting on-time performance ahead of standard safety practices, including regular track maintenance. In October, Schumer called it a "horror house of negligence."

The MTA has embraced most of the recommendations made by the NTSB and other federal officials and safety experts and instituted several safety upgrades, including increased track maintenance, "alerter" systems in locomotive cabs to ensure engineers are awake, and a pilot project to screen engineers for sleep disorders.

The MTA also has expedited a federally mandated plan to install "positive train control" crash-prevention technology on Metro-North and the Long Island Rail Road.

But some concerns have persisted, including when a train derailed outside of Grand Central just last Wednesday.

However, with modern grade crossing gates designed to "fail safe" and come down under their own weight during any disruptions, the vast majority of grade crossing accidents occur "because motorists choose to ignore warnings signs, signals, or safety gates," according to the Federal Railroad Administration.

About 270 people die each year in grade crossing accidents, although that number has dropped by more than 50 percent over the last 20 years.

Modern grade crossing gates are also built with "black box" event recorders that can determine whether gates functioned properly during an accident -- including if they came down, when they came down, and for how long were they down.