An FDNY official peers from an opening near the top...

An FDNY official peers from an opening near the top of 261 Madison Ave., which was damaged when a crane dropped an AC unit May 31, 2015. Credit: AP / Craig Ruttle

A crane hoisting an air-conditioning unit to the top floor of a 30-story midtown Manhattan office building dropped the massive chiller, which slammed into the building and plunged to the street, leaving seven people injured Sunday morning, officials said.

Workers were guiding the crane to the top floor of 261 Madison Ave. between 38th and 39th streets shortly before 11 a.m. when the air conditioner fell, startling neighbors and passersby who reported hearing a loud bang and seeing thick smoke, officials said. The unit left a gaping hole in the building's exterior and sent glass and jagged metal raining onto Madison Avenue, officials said.

The seven injuries were reported as "minimal to moderate," officials said. Two construction workers were taken to Bellevue Hospital Center, treated and released. Two people who were in a cab that stopped short at the scene were treated and released at NYU Langone Medical Center, a hospital spokeswoman said. Three people refused medical treatment at the scene.

Mayor Bill de Blasio, who arrived at the site with other city officials Sunday afternoon, said the vast debris field indicated more people could have been injured if the incident had occurred during the busy workweek.

"Thank God no one was seriously injured," de Blasio said at a news conference near the damaged building. "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, officials said. The city's Department of Buildings is investigating whether the incident was caused by operator error or a mechanical malfunction, said a source briefed on the matter.

The buildings department approved the installation of a mobile crane at the site on May 4, officials said. At the time of the accident, there were no open complaints or any open Environmental Control Board violations relating to safety issues, officials said.

A representative for The Sapir Organization, which owns 261 Madison Ave., said the company is 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 said in an email.

It was unclear whether the crane that was hoisting the unit had passed its most recent inspection, said Rick Chandler, commissioner of the city's building department, but it "looks structurally sound."

"From what I can tell, the crane was in good working order," Chandler said. "We do track all cranes around the city . . . we'll be investigating that specific device as thoroughly as possible."

The drama began at 10:43 a.m. with the first of several 911 calls, FDNY Assistant Chief Ronald Spadafora said.

Eileen Travers, 47, was at home nearby with her two teenage daughters when the crane dropped the air-conditioning unit. She said she heard a boom, went to her window and could see nothing but smoke and debris.

"You couldn't see beyond the corner," Travers said.

The air-conditioning unit shattered and landed on Madison Avenue, which was open to only one lane of vehicle traffic on that block as required when a crane is being operated, officials said.

Several blocks around the scene were closed Sunday as workers cleaned the area of broken metal, fiberglass and glass. By 3:45 p.m., the crane was loaded onto a flatbed truck and driven away.

De Blasio said city officials hoped to have the busy midtown thoroughfare opened by the morning rush hour.

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