Madison Avenue between 38th and 39th Streets remained closed on Monday morning, one day after seven people were injured when an air-conditioning unit being hoisted by a crane 30 floors above midtown hit a building and then fell to the ground.

Workers were lifting the AC unit up to the top floor of 261 Madison Ave. at about 10:45 a.m. when it dropped, causing debris to fall down the west side of the building, between 38th and 39th streets.

The injured included two construction workers who suffered minor injuries, said an FDNY spokesman.

"Thank god there are no life threatening injuries," Mayor Bill de Blasio said to reporters at the scene. "There were pedestrians passing by and luckily it occurred during an hour of the day when not too many people are around."

The cause of the accident was unknown Sunday and De Blasio promised "a full investigation to understand what happened here, and to guard against anything like this in the future."

Two of the civilian victims, who were in a cab that stopped short when the debris started falling, were taken to NYU Langone Medical Center where they were treated and released, said a hospital spokeswoman. The two construction workers were taken to Bellevue Hospital Center and three civilians refused medical attention at the scene, an FDNY spokeswoman said. 

The construction workers were both released, said a Bellevue spokeswoman.

At least one car was damaged as well, said Ronald Spadafora, an assistant chief with the FDNY, during a news conference from the scene of the incident. Spadafora said the incident could have been worse, adding "we lucked out on this."

Officials evacuated several buildings in the area and closed off streets. Many who lived and worked nearby gathered by the police lines, peering at the debris and snapping photos.

Eileen Travers, 47, was at home on Madison Avenue with her two daughters when they heard a loud boom. They immediately ran outside and were confronted with a plume of smoke.

"You couldnat see beyond the corner," Travers said. 

The building includes the upscale Japanese restaurant Zuma, which was closed at the time of the accident, and WeWork, a rentable office space.

It was unclear if the crane had passed its most recent inspection, said Rick Chandler, the commissioner of city's Department of Buildings, but it "looks structurally sound."

The building has been under construction since February, Chandler said, and there have been no complaints filed.

A representative for the crane's operator, Bay Crane, declined to comment on Sunday. 

A Department of Buildings spokesman declined to comment on the company and said the incident was under investigation.

A representative for The Sapir Organization, which owns 261 Madison Ave., said they are assessing the damages and working closely with city agencies.

"Our first priority remains people's safety, ensuring no more injuries occur and that the building will be safe and operational," the company wrote in an email.

(with Maria Alvarez and Emily Ngo)