(CNN) — Federal transportation investigators will work to determine what caused two passenger trains to collide during rush hour in Connecticut, sending dozens to the hospital.
Officials from the National Transportation Safety Board will be at the site of the crash this morning, the agency said.
The two Metro-North passenger trains, heading in opposite directions, collided Friday evening in southwestern Connecticut, damaging both trains and leaving some people critically injured.
The accident occurred when a train heading from New Haven to New York City derailed around 6:10 p.m. That train struck the other train in Bridgeport, Connecticut, Metro-North spokeswoman Marjorie Anders said.
Gov. Dannel Malloy told reporters Friday night that five people were "critically injured," one of whom was in "very critical condition."
By Saturday morning, one of the two patients who had been in critical condition at Bridgeport Hospital had been upgraded to fair condition, hospital spokesman John Cappiello said. Of the total patients at the hospital, 10 have been released.
St. Vincent's Medical Center, also in Bridgeport, treated 44 patients from the incident, hospital spokeswoman Lisa Sanzone said. One of those was in critical condition. A total of 36 patients have been released from the hospital, she said.
By early Saturday, 46 of the 70 people who had gone to hospitals had been released.
A passenger in a middle car of the New York-bound train, Chris Martin, said his car went dark after the crash.
He then heard someone yell over the intercom for "all the doctors up front."
Martin said his crowded train was evacuated. Everybody on his train was physically fine, he said, but many were shaken emotionally. He said he saw injured people outside the train.
Brian Alvarez said he saw the wreckage.
"I saw this one car and it was completely destroyed, and they were pulling people out of the car," Alvarez said. "... They were all bloody."
Power was shut off along the line and service has been halted in the area.
Amtrak also announced Friday night that it had suspended all travel between New York and Boston indefinitely after the crash. Bridgeport Mayor Bill Finch said such travel headaches could persist for weeks because the two tracks affected by the derailment -- which are both "shot right now" -- may take weeks to repair.
Because of a bridge replacement project, those two tracks are the only way in and out of New York City by train from that part of Connecticut.
"This is our pipeline to New York City, and it's going to be shut down for some time," the mayor said.