The FDNY said it was an "electric blowout" that caused a three-alarm fire that destroyed decades of DNA evidence at an NYPD warehouse in Brooklyn late last year.

A source familiar with the investigation said the warehouse was still being powered by a backup generator over a decade after the building's electrical grid was "heavily damaged" during flooding from Hurricane Sandy, a contributing factor in the December "blowout."

Fire marshals determined the blowout started “in a conduit leading to an exit sign,” the FDNY tweeted on Thursday morning. An FDNY spokesperson declined to elaborate on the marshal’s findings.

Asked about the decade-long delay in improving the warehouse’s electrical grid, an NYPD spokesperson said construction on a project to fix it “was about to commence” at the time of the fire.

While the NYPD declined to respond to questions about how evidence was stored at the Red Hook Erie Basin Auto Pound, accounts have emerged of materials stuffed into cardboard bags and packed together in cardboard drums. Fire officials at the scene in December described a “large amount of combustible material” that meant it took them more than a day to stamp out the embers.

The same warehouse flooded during Hurricane Sandy in 2012, sparing barrels stored on higher shelves but ruining evidence on lower ones. Anything that survived the floodwaters was likely destroyed by last year’s fire.

Just eight barrels of evidence stored in the sprawling warehouse were spared in the inferno, an NYPD spokesperson said Thursday. The NYPD said they are working with the district attorney's offices “on a case-by-case basis to verify the location and status of each specific evidence request,” the spokesperson added.

Experts have said the loss will most deeply affect cold cases and wrongful convictions, both of which rely on the ability to dig up old evidence and re-examine it.

Elizabeth Felber, an attorney at the Legal Aid Society’s Wrongful Conviction Unit, said they were still waiting for word from the NYPD or district attorneys' offices about which cases might be affected.

“Clearly the laws need to be strengthened so every [10] years we don't have another biblical disaster,” she said. “They need to store them in fireproof containers, maybe not keep it in a rickety, old warehouse.”

The fire had already affected one of her clients’ cases, though she wasn’t authorized to speak on the specifics, Felber said. They’d been told the evidence they were looking for had most likely been stored at Erie Basin.

“That man is serving his 36th year of incarceration,” she said. “They need to take their jobs seriously. These are people’s lives.”

This story has been updated with comment from the NYPD.