The Boston subway car that caught the arm of a passenger and dragged him to his death last month had a faulty mechanism in a door that “short-circuited,” federal and local authorities reported Monday.

Robinson Lalin, 39, was getting off a Red Line train at Boston’s Broadway Station at 12:30 a.m. April 10 when his right arm got stuck in the passenger door as the train departed, the National Transportation Safety Board (NTSB) and local authorities said.

Lalin was pulled 105 feet along the platform and onto a lower level near the tracks, where he was later found with injuries. He died at the scene.

The NTSB, which has been investigating the incident for the past three weeks, wrote in a preliminary report released Monday that a faulty mechanism in a door control system allowed the train to move with the door obstructed, which the train’s safety features are normally supposed to prevent.

The Massachusetts Bay Transportation Authority, which operates the Red Line, in a statement Monday identified the problem in the door mechanism as a “short circuit.”

“He was literally slaughtered to death,” the victim’s nephew, Kelvin Lalin, told WBEZ while holding a vigil Monday evening outside the subway station where the dad of two died last month. “My uncle did not deserve that.”

Kelvin said his family is devastated after reading the NTSB report detailing what happened.

“I can imagine him screaming,” the nephew added, referring to his doomed uncle. “I can imagine him being scared, so scared, panicking.”

Kelvin argued that the accident that claimed Lalin’s life was preventable and resulted from negligence.

In a statement, the MBTA said the NTSB has confirmed the transit authority’s initial assessment of a short circuit in the car’s wiring that allowed the train to begin moving while Lalin was trying to leave.

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 MBTA said.

The railcar remains out of service and the train operator is on leave.

The investigation is ongoing and Monday’s report could change, the NTSB said. The federal agency has so far examined and tested the train equipment, reviewed security video, observed train operations, conducted interviews, and performed sight-distance observations.

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 broke down, and more than two dozen went to the hospital last July when one Green Line train rear-ended another.  

The MBTA’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.

With Post wires