It was not immediately clear what caused the derailment, but the authority said that the police were investigating “as though it were a crime scene.” The National Transportation Safety Board said that it was dispatching a team to investigate the derailment.

The crash also obstructed operations for Amtrak, which suspended service between New York and Boston. The transportation authority said Metro-North service was suspended between South Norwalk and Bridgeport.

After arriving at the scene on Friday night, Gov. Dannel P. Malloy of Connecticut said that of the five serious injuries, one person was in “very critical” condition. He added that he had inspected the trains, and saw that the siding of one car had been ripped off. Mr. Malloy estimated that train service would return on Monday at the earliest.

The authority said the collision occurred in an area where two of the four tracks were already out of service for work on overhead wires, suggesting that service disruptions could persist.