A faulty fan caused the MTA’s security cameras at a Brooklyn subway station to malfunction the day before a madman unleashed gunfire on straphangers there last month, agency officials have revealed.

NYPD sources have said the lack of crucial video footage during the mayhem slowed their hunt for accused gunman Frank James, who, unbeknownst to them, had escaped amid the chaos on a train across the platform.

The glitch also shut off feeds at the 25th Street and 45th Street subway stations, where James entered and exited the transit system — and then remained on the lam for more than 24 hours before being nabbed and charged in the April 12 carnage.

Transit crews had been working since April 7 to fix the fan issue, MTA CEO Janno Lieber wrote in a letter late Monday to congressional representatives demanding answers. He said the actual surveillance feed remained unaffected until April 11.

The cameras’ surveillance feed cut out “less than 24 hours” before James allegedly fired 33 shots into a subway car pulling into the platform, wounding 29 people, Leiber said.

“Technicians replaced the fan unit on the morning of April 8, but the network diagnostics still indicated a problem,” Lieber wrote. “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 streaming failed.”

Lieber said technicians were working on the feed at the station at the time of the shooting and were then instructed by the NYPD to leave amid the shooting.

MTA “networking specialists” took the new equipment offsite and reinstalled it at the station “by 12:30 p.m. on April 13,” Lieber wrote.

Lieber and his team have defended the camera malfunction as a rare occurrence offset by 36 other MTA video feeds used by the NYPD during its investigation into the shooting.

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“On any given day, we have approximately 99 [percent] availability of our subway station security cameras,” Lieber wrote in the letter to US Reps. Bennie Thompson (D-Miss.), John Katko (R-NY) and Ritchie Torres (D-NY).

The three reps had demanded answers as to why the cameras failed during the shooting, noting a recent $5 million funding boost in transit security funds for New York passed by Congress.

Lieber in his response said those funds were “appreciated” but insufficient.

The MTA needs $1.3 billion to “further expand and upgrade” its surveillance system but currently has just $300 million available, he said.

NYPD Deputy Commissioner John Miller said in a statement to The Post the week of the subway attack that, “The cameras were out at three stations due to a technical issue.

“Statements that the lack of cameras on the station delayed the manhunt by many hours are unfair and misleading. We had witness descriptions of the suspect and the distinctive, bright colored clothing he wore during the attack.

“The MTA cameras in other parts of the system were essential elements in determining his movements before and after the shootings,” Miller said. “Their personnel worked with us around the clock to identify and retrieve images in this case. While it has become routine to cast blame in many directions after an incident we should remember that the gunman is the sole party responsible for this attack.”