Why the cameras weren't working during the NYC subway shooting

A faulty fan caused the glitch that prevented security cameras at a Brooklyn subway station from transmitting during a mass shooting on a rush-hour train last month, according to the Metropolitan Transportation Authority.

The cameras were working until “less than 24 hours” before the April 12 shooting on the N train as it approached the 36th Street station in Brooklyn's Sunset Park neighborhood, MTA Chair Janno Lieber wrote in a letter to congressional representatives obtained by ABC News.

In the days before the incident, technicians worked to replace the fan unit for an issue that initially wasn't impacting the transmission of the cameras' feed, according to Lieber.

"Technicians replaced the fan unit on the morning of April 8, but the network diagnostics still indicated a problem," Lieber wrote in the letter, dated May 2. "MTA technicians made a series of repairs in an effort to correct the issue, and on the morning of Monday April 11, as technicians were installing new communication hardware, the camera failed."

Lieber characterized the cause of the outage as a "failure of hardware and software" at the communications room that governs the station's cameras that prevented them from transmitting their feed. The outage also impacted the cameras at the 25th Street and 45th Street subway stations.

"Technicians were working in the communications room on the next morning, April 12, when the attack took place," the chair wrote. "NYPD directed them to leave the communications room as the investigation began."

The cameras were back online by 12:30 p.m. on April 13, according to Lieber.

Dozens of people were injured, including 10 by gunfire, in the shooting. Police arrested a suspect more than 24 hours after the incident.

The NYPD, which had initially said the cameras were out at the three stations due to a "technical issue," called claims that the lack of operating cameras delayed the manhunt "unfair and misleading."

"The MTA cameras in other parts of the system were essential elements in determining his movements before and after the shootings," John Miller, deputy commissioner of intelligence and counterterrorism for the NYPD, said in a statement in the wake of the attack.

In his letter, Lieber also said the MTA's subway camera system played a "critical role in the manhunt."

The alleged gunman, 62-year-old Frank James, faces a terror-related count. Last week, defense attorneys charged in a court filing that federal agents improperly questioned him. In response, the federal government said it was authorized and within its rights in its interactions with James.