New York (CNN) -- Rail cars flipped over when a passenger train derailed Sunday morning, killing at least four people and injuring dozens more.
Firefighters and emergency rescuers swarmed the scene near Spuyten Duyvil station in the Bronx, where at least two train cars turned on their sides as they skidded off the tracks. One car was just feet away from the Harlem River.
Three of the dead were thrown out of the train as it "came off the track and was twisting and turning," New York Fire Department Chief Edward Kilduff told reporters.
"The windows broke out. ... The gravel came flying up in our faces," said passenger Amanda Swanson, who put her bag in front of her face to block the rubble.
"I really didn't know if I would survive," she said. "The train felt like it was on its side and dragging for a long time. ... The whole thing felt like slow motion."
Police divers were in the water hours after the crash looking for survivors, and cadaver dogs searched the wreckage. Authorities believe all the passengers have been accounted for, New York Gov. Andrew Cuomo told reporters.
It was unclear how fast the train was traveling and how many passengers were on board.
"I heard this horrible, whooshing sound. ... It was very disturbing, very loud," said Hank Goldman, who lives near the tracks. "I jumped out of bed and looked out the window and I saw a light-colored object lying down. I thought it was the roadway to the train. Then I got my binoculars, and I couldn't believe my eyes, that the train had jumped the tracks right here."
Investigators comb the scene
At least 67 people were injured, 11 seriously, said Joe Bruno, New York's commissioner of emergency management.
"In terms of causes, we don't know exactly what happened," Cuomo said.
Federal investigators from the National Transportation Safety Board were headed to the scene, he said.
The train operator -- who is among the injured -- told investigators he applied brakes to the train, but it didn't slow down, a law enforcement official on the scene and familiar with the investigation said.
"That will be a key point of concern, whether this train was moving too quickly," Bruno said.
Investigators will look at a number of details, including the track condition, the signal system and human factors, NTSB Chairwoman Deborah Hersman said.
They'll also search for data and video recordings that may have captured details tied to the derailment, she said.
"We've got a lot of work to do," she said. "We don't have a lot of daylight hours to do it, but we're going to do as much as we can."
Passenger: Train stopped with a thud
The train had been traveling from the Hudson Valley town of Poughkeepsie, 100 miles north of New York, to New York's Grand Central Station.
It came off the tracks just as it was coming around a sharp curve shortly after 7 a.m., fire officials told CNN affiliate WCBS.
Of eight train cars, seven were off the tracks.
Passenger Frank Tatulli told WABC he thought the train was traveling "a lot faster" than usual.
He escaped a derailed car on his own and had head and neck injuries, he said. Other passengers were still on the train, he told WABC.
Another survivor told WABC that she climbed out of a train car that had overturned.
Nearby, she said, she heard injured victims moaning and asking for help.
"I almost feel guilty," she said. "I was just in a really safe spot on the train, just the way it fell."
Swanson told CNN the train car she was in came to a stop with a thud.
"I just closed my eyes and kind of hoped to God that I was going to be able to call my mom with decent news," Swanson said.
She got off the train with her cell phone in hand.
"The screen was shattered, but it still worked," she said, "and I just immediately called 911."
Hospitals treat wounded; rail lines shut down
Injured victims were taken to several hospitals in New York.
At St. Barnabas Hospital, doctors treated 12 patients, at least two of whom were in critical condition. One of the victims suffered a spinal cord injury that could leave him paralyzed from the neck down, said Dr. David Listman, director of the hospital's emergency department.
New York-Presbyterian Hospital said it was treating 14 patients, four of whom were in critical condition.
The derailment occurred near the area where a freight train carrying trash derailed in July. No one was injured in that accident.
Federal authorities are also investigating a collision between two Metro-North trains in May, when two passenger trains crashed during rush hour in Connecticut.
Service was suspended Sunday on part of the Hudson Line, Metro-North said on Twitter.
Amtrak said it was suspending service between New York and Albany indefinitely after the derailment.
It's unclear how long the investigation will take, said Aaron Donovan, a spokesman for Metro-North Rail.
"Once the NTSB gives us the all-clear, then we can begin to repair and clean up the tracks," he said.
Asked if there's any indication terrorism was involved in Sunday's derailment, a national security official told CNN "none that we know of."
U.S. President Barack Obama was briefed on the derailment.
"His thoughts and prayers go out to the friends and families who lost a loved one and everyone affected by this incident," the White House said.
Sen. Charles Schumer of New York called on federal authorities to investigate "with speed and certainty."
"We must figure out how this happened," he said.