The city Department of Investigation is probing the elevator accident that fatally crushed a 30-year-old man as he tried to step into the lobby of his Manhattan apartment building, the agency said Friday.

The gruesome death of Sam Waisbren came weeks after the Department of Buildings found that a safety device on the other elevator in the Manhattan Promenade tower was “tampered with” and “rendered inoperative,” records show.

The May 29 discovery led to a Class 1 Environmental Control Board violation and an unpaid $1,280 fine over the “immediately hazardous” condition, according to the DOB website.

A spokeswoman who confirmed the DOI probe didn’t detail its focus or potential targets.

The DOI is responsible for “investigating and referring for criminal prosecution cases of fraud, corruption and other illegal activities by City employees, contractors and others who do business with the City,” according to its website.

“DOI’s strategy attacks corruption comprehensively through systemic investigations that lead to arrests, and recommendations that result in reforms and preventive controls that improve the way the City operates and delivers services to all New Yorkers,” the site says.

Waisbren was killed when the elevator abruptly plummeted as he tried to follow another man off it around 8:15 a.m. Thursday, in a horrific incident captured on surveillance video exclusively obtained by The Post.

Elevator-safety expert Kevin Doherty, who watched the video several times, has told The Post that the odds were “almost incalculable” that it wasn’t the result of human error.

A DOB spokesman said inspectors went to the 23-story, luxury rental building at 344 Third Ave. on Friday to check out the elevator and the elevator room.

The DOB inspection will determine whether the deadly mishap was “a one-off” or something that would require inspections of other elevators citywide, spokesman Andrew Rudansky said.

The elevator that killed Waisbren was installed in 1997 and was never cited for any safety-related violations, Rudanksy said.

The elevator to its right, which was found with a disabled “door zone restrictor” on May 29, was repaired two days later and returned to service following a DOB inspection, he said.

The DOB website continues to list the violation as open because the building owner, Manhattan Promenade LLC, hasn’t paid the fine and filed necessary paperwork, Rudansky said.

An additional, $1,500 fine was imposed on Thursday because 90 days had passed since the violation was issued, Rudansky said.

That fine wasn’t related to the accident that killed Waisbren, he said.

The surveillance video shows the elevator that is the subject of the fines was out of service when Waisbren was killed, and Rudansky said that was because it was undergoing “routine maintenance.”

It was back in service Thursday evening — but several tenants on Friday said they were too terrified to use that elevator and were instead climbing the stairs.

“I don’t think I’ve seen a single person take the elevator since it happened,” said Sonya, a 27-year-old public-relations worker who lugged a carry-on suitcase down from her 18th-floor apartment.

“Really, I’m freaked out by the whole thing. Even the elevator at work is making me jumpy.”

A 30-year-old man who lives on the fifth floor emerged from the stairwell with a full set of Ping golf clubs.

“Right now I’m feeling things out. But I probably won’t take the elevator for at least another month,” he said.

Several delivery workers also said they dropped all their packages at the front desk rather than risking a ride.

Property records identify Manhattan Promenade LLC’s manager as Robert Kalimian, who’s also the managing partner and chief of operations of ATA Enterprises Management, which manages the building.

No one answered the phone at ATA’s office on Friday, and Kalimian didn’t return messages left on phones listed in his name.

An email sent to building residents by ATA early Friday afternoon said, “There was an incident involving one of the elevators on August 22, 2019.”

“Out of respect for the privacy of the parties involved, and while the matter is being investigated by the appropriate authorities, we will have nothing further to add,” the message added.

Additional reporting by Alex Taylor and Kevin Fasick