US officials opened an investigation Saturday into a rush-hour collision between two commuter trains that injured more than 70 passengers and severed a key rail link.

Train service in the busy Northeast corridor from New York City to New Haven, Connecticut used by tens of thousands of commuters was suspended indefinitely and will likely be snarled for days as officials seek to determine the causes of the crash.

Investigators from the National Transportation and Safety Board were focusing on a section of the rail on the eastbound track going toward New Haven that was fractured at a rail joint.

"This fracture is of interest to our investigators because (of) some of the witness marks on the fracture surface of the rail," NTSB board member Earl Weener told reporters, stressing that it was unclear whether the fracture occurred before or after the accident.

Workers were inspecting cars involved in the derailment, have obtained inspection documents and were seeking maintenance records. The trains belonged to Metro-North, one of America's busiest commuter rail lines, carrying around 280,000 passengers a day.

It said one of the two tracks in the affected area suffered "extensive infrastructure damage" due to the collision, with both the track and overhead wire damaged.

Investigators, meanwhile, secured the site of the crash in Fairfield, Connecticut, bringing traffic to a halt until at least sometime Monday.

Officials were unable to say how long it would take to clear and repair the tracks. The cause of the accident was not immediately known, but Connecticut Governor Dannel Malloy told reporters Friday there was "no reason to believe that this was anything but accidental."

Eight people remained hospitalized, three of them in critical condition, Malloy said on his Twitter account.