The Red Line door that clamped down on a man’s arm and caused the death that prompted last year’s federal investigation into the MBTA failed because of a short circuit, the National Transportation Safety Board states in a new report.

An NTSB investigation determined that a short circuit in the “passenger door interlock circuit” allowed the train to thrust forward while a man, 39-year-old Robinson Lalin, was obstructing the door of one of the railcars on April 10, 2022.

Lalin had been attempting to exit the train from the second of six railcars, but was dragged and killed as a result of the electrical failure, Tuesday’s report states, citing surveillance footage reviewed during the investigation.

“According to the video, the doors closed on the accident passenger’s upper body on his right side as he stepped back onto the platform,” the report states. “As the train accelerated to depart the station, the passenger ran alongside the train with his upper body pinched between the doors.

“The train did not stop, and the passenger collided with a wall at the end of the platform and was killed.”

The passenger door interlock circuit system for this particular series of MBTA railcars, the 1500, was designed to prevent such an occurrence, by disabling train movement should a passenger door be open on any of the cars, the report states.

The railcar passenger doors were also designed to become secure in their positions when train speeds reached 3 mph.

When this Red Line train accelerated to leave Boston’s Broadway station, “it quickly reached 3 mph, and the doors became secure in their positions, leaving the passenger unable to free himself,” NTSB states in the report.

The MBTA expects to retire the 1500 series of railcars from service by March 2024, the report states. These cars were put into service between 1969 and 1970, a T spokesperson said.

While the short circuit was listed as the “probable cause,” in the report, NTSB also found the train operator had violated departure procedures, by “pulling her head back into the operating cab before the pilot lights above the doors had turned off.”

Investigators also identified a 19-foot blind spot on a station camera monitor. Had the train operator been using this camera to check whether the platform was clear before leaving the station, “it was unlikely that she would have seen the accident passenger stuck in the doors,” the report states.

The train operator was hired by the MBTA in October 2018. She was disciplined by the agency for an incident that involved a prior door failure on Dec. 30, 2019.

She also received disciplinary action for failing to stop for a double red signal on Oct. 17, 2021, and completed reinstruction training on the proper procedure to stop in November 2021, NTSB said, citing a review of employment records.

The operator, who was not identified in the report, did not have any drugs or alcohol in her system when the incident occurred, NTSB said.

The Federal Transit Administration launched a rare safety investigation of the MBTA’s subway system three days after the Red Line fatality. It was only the second time such a probe had occurred. The Washington Metropolitan Transit Authority was the first U.S. subway system to be investigated.

MBTA spokesperson Lisa Battiston said the agency was “closely reviewing” the NTSB’s final report, “and expresses its appreciation for the NTSB’s diligent work on the circumstances surrounding the tragic death of Robinson Lalin at Broadway station.”

“The MBTA would once again like to extend its deepest condolences to the Lalin family regarding this heartbreaking incident,” Battiston said in a statement.

Battiston said the report confirms the T’s initial assessment of a short circuit in one of the Red Line car’s wiring that allowed the train to begin moving while Lalin was attempting to exit through the closing doors.

Door systems throughout the Red Line fleet were tested for this specific problem “immediately following the accident.” A similar circuitry problem was not found in any of the other Red Line cars of the same make and model, nor in any other subway car, all of which were tested, Battiston said.

Additional door inspection protocols added since the incident, as part of “regularly scheduled preventative maintenance,” have not identified a similar circuitry problem in “any other vehicle throughout the fleet,” Battiston said.

Battiston also noted that train operators receive extensive training and instruction on safe operations.

Since the April 2022 fatality, the T has performed a system-wide audit of camera monitors, mirrors, signage, platform lighting, and cameras on the Red, Blue and Orange Lines to ensure operators “are seeing clear and unobstructed views of berthed trains on the platforms, Battiston said.

She said 31 of 55 monitors have been upgraded, including the two that were at Broadway station at the time of the fatality.

“The safety and well-being of every MBTA rider is of the utmost importance,” Battiston said. “The MBTA has been working aggressively to improve safety at all levels and has been advancing safety-related objectives with billions of dollars in infrastructure and vehicle investments in recent years.”

Lalin’s family filed a lawsuit against the MBTA in March, alleging the agency’s “carelessness and negligence” resulted in his death.

Attempts to reach the family members listed in the suit were unsuccessful and their attorney did not return a phone call seeking comment.