Officials work at the site of a derailed Amtrak train in Philadelphia, Pennsylvania May 13, 2015. REUTERS/Mike Segar

Rescue workers on Wednesday searched through twisted metal and debris after an Amtrak train derailed in Philadelphia, while investigators sought the cause of the accident that killed at least six people and injured scores of others.

Authorities said they did not know why the New York City-bound train carrying 243 people derailed at about 9:30 pm Tuesday (0130 GMT Wednesday), sending all seven cars and the engine off the track. One car was tossed upside down and three on their sides, and passengers and luggage were sent flying, survivors said.

Philadelphia-area hospitals and health systems reported treating about 135 people. Philadelphia Mayor Michael Nutter said authorities had not yet accounted for everyone aboard the train.

One passenger, Daniel Wetrin, told CNN by phone that he was riding in the last car when it derailed and he was thrown into the aisle.

"Chairs were flying around, people were flying, bags, (it was) pretty chaotic," he said. "There were two people above our heads in the luggage rack."

Moments later, passengers in his car managed to open a rear door and escape, he said, adding he was unaware of any major injuries.

In a video posted on social media, passengers could be heard crying while others told victims to crawl forward.

"It's an absolute disastrous mess," Nutter said. "I've never seen anything like this in my life."

The crash of Amtrak train No. 188, en route from Washington, D.C. with a crew of five, was the latest in a series of rail accidents on heavily traveled passenger train routes over the past year.

The train derailed in the city's Port Richmond neighborhood along the Delaware River, near the site of a 1943 rail accident that killed 79 people.

Amtrak, a publicly funded national passenger rail line, provided no further details about the accident. It said Amtrak service along its busy Northeast corridor between New York and Philadelphia had been suspended.

Nutter said he doubted the service would be restored through Philadelphia this week.

CNN quoted the Federal Bureau of Investigation as saying there was no indication that terrorism was a factor in the derailment. Three law enforcement officials told Reuters they had not encountered anything suspicious.

"We do not know what happened here. We do not know why this happened," Nutter told reporters. He later said on CNN there was no indication there had been a collision with another train.

The National Transportation Safety Board said a "go-team" that would investigate the crash was expected to arrive on Wednesday morning.

The crash and the issue of Amtrak funding are likely to come before Congress later Wednesday, when the House Appropriations Committee meets to discuss the transportation budget for the next fiscal year.

White House spokesman Josh Earnest told CNN's "New Day" program that more investments in rail infrastructure is "common sense."

"There is clearly more that can be done when we're talking about a railway infrastructure that is decades old," Earnest said.

"ALL OF A SUDDEN ... OFF THE RAILS"

Former Pennsylvania Congressman Patrick Murphy, who was a passenger on the train, told MSNBC the cafe car he was riding in flipped over, but he escaped with minor cuts and bruises.

He estimated the train was traveling at about 60 to 70 miles per hour (96-113 kph) when "all of a sudden, it went off the rails."

In the immediate aftermath of the crash, the area was filled with live electrical wires, Murphy said.

"I get most choked up when I think of the first responders who risked their lives to help save the passengers," he said.

Dr. Herbert Cushing, Temple University Hospital's chief medical officer, said the dead suffered massive chest injuries. Most of the injured at Temple suffered fractures, he said at a news conference.

Port Richmond is a working-class neighborhood that has recently become a popular place to live among younger adults in the city.

Michael Hand, 44, who lives a few blocks from the crash, said he was outside drinking a beer at the time.

"There was a flash and then there was a big boom," he said.

In March, 21 people were injured in Los Angeles when a commuter train collided with a car. A month earlier, 50 people were hurt and an engineer fatally injured when a Los Angeles-bound Metrolink train struck a pickup truck.

In February, six people were killed and a dozen injured when a Metro North commuter train north of New York City hit a car stalled on the tracks. The driver of the vehicle also died.

The Philadelphia accident occurred near the site of a train derailment at Frankford Junction in north Philadelphia, killing 79 people and injuring 117 others in 1943, according to the National Railway Historical Society.