BOSTON — A passenger door on a Boston subway car did not function properly when a man got his arm stuck in it and was dragged to his death last month, federal investigators said Monday.

The trains are equipped with safety features to prevent them from moving when the doors are obstructed, the National Transportation Safety Board wrote in the preliminary report.

“NTSB investigators examined and tested the railcar involved after the accident, identifying a fault in a local door control system that enabled the train to move with the door obstructed,” the report said.

The MBTA in a statement Monday identified the problem as a “short circuit.”

The man, identified by local authorities as Robinson Lalin, 39, of Boston, died around 12:30 a.m. April 12 as he exited the six-car Massachusetts Bay Transportation Authority Red Line train at the Broadway Station.

Lalin got his right arm stuck in the door and was dragged more than 100 feet along the platform, onto a lower surface near the tracks, the NTSB report said. He was pronounced dead at the scene.

“The NTSB has confirmed the MBTA’s initial assessment of a short circuit in the car’s wiring that allowed the train to begin moving while Mr. Lalin was attempting to exit through the closing doors,” the MBTA’s statement said.

The MBTA inspected the doors on the other railcars and did not find any similar problems.

“During rigorous testing, the problem with the incident car could not be duplicated in any of the other Red Line cars of the same make and model,” the T said.

The railcar and the train operator remain out of service.

The investigation is ongoing. The NTSB has so far examined and tested the train equipment, reviewed security video, observed train operations, conducted interviews, and performed sight-distance observations.

The NTSB said Monday’s report is preliminary and subject to change.

There have been other safety issues with the MBTA in the past year. Nine people were injured in September when an escalator at the Back Bay Station malfunctioned, and more than two dozen went to the hospital last July when a Green Line train rear-ended another trolley.

The T’s statement said safety is a priority and the agency has spent $8 billion in infrastructure and vehicle investments over the past five years and almost doubled the size of its safety department in the past three years.