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Protesters gather outside of the Metropolitan Detention Center in Sunset Park, Brooklyn, which didn't have heat or electricity for over a month.

Federal authorities left inmates at the Metropolitan Detention Center in Brooklyn without heat as temperatures nosedived last winter — and had no plan in place to alert higher-ups of the system meltdown, a Justice Department probe found.

The report, issued Thursday, said investigators found “significant heating issues” at the federal facility in Sunset Park exist even under normal conditions at the lockup.

The federal probe into the prison, which houses about 1,600 inmates, started on Feb. 7 after shocking lawsuits and media reports detailed the horrid conditions inside the facility.

Investigators said staffers “long struggled” to regulate — or even measure — temperatures in the winter before the system entirely failed between Jan. 27 and Feb. 3.

Wild swings were most common in the facility’s West Building, where temperatures range from 58 degrees to above 80.

“We found that facilities staff used an unreliable method to measure West Building temperatures before and during the power outage,” the report said.

“As a result, we cannot state with reasonable assurance how many inmate housing areas experienced temperatures at or below the BOP target of 68 degrees or for how long localized temperatures remained below.”

News trickled out of the conditions after a Jan. 27 fire knocked out a third of the jail’s electrical infrastructure.

Following the fire, the prison prohibited attorneys from coming in and canceled social visits instead of taking steps to restore the system temporarily, the report says.

While the DOJ found it wasn’t “unreasonable” to cancel the visits at first, investigators faulted the staff for not “having an alternative visitation plan available to use once it became clear that the power outage would not be resolved promptly.”

The federal report gives a list of changes to fix the current issues.

The prison became the subject of national media attention once the Federal Defenders of New York, a group of attorneys who defend the inmates, filed a lawsuit. The suit called the conditions a “humanitarian crisis” and said their clients are being subjected to “inhumane” conditions that violate their civil rights.

The lawsuit was dismissed, but a separate class action class action suit regarding the lack of heat at MDC is still pending.

A spokesperson for the Bureau of Prisons did not immediately respond to a request for comment.

Additional reporting by Andrew Denney