An investigation into what caused an air conditioning unit to fall from a crane in midtown and injure 10 people, was under way yesterday.

The investigation is looking into the history of the Long Island City-based crane company and the building, a city Department of Buildings spokesman said. They are also looking at how well the AC-unit was strapped to the crane.

"We're obviously looking for anything that we need to know here that might change our policies," Mayor Bill de Blasio said Monday at an unrelated news conference. "Look, sometimes it is a situation where something happened that was truly very narrow and specific to that one incident, but I'm not going to conjecture until we get the investigation back."

The 23,000-pound commercial air conditioner unit fell 30 stories from the top of the Madison Avenue building, between 38th and 39th streets, at about 10:45 a.m. on Sunday, according to the mayor's office. It was being hoisted onto the roof at the time.

The falling debris injured five civilians -- two of whom were in a cab at the time -- two construction workers, two NYPD personnel and one firefighter. All suffered minor injuries, an FDNY spokesman said.

The large unit hit the building on the way down, damaging the facade, according to the mayor's office.

A small break in the 12-inch water main in front of the building was then discovered, and the Department of Environmental Protection was repairing the break and assessing the extent of the damage Monday.

The crane was removed on Sunday night, officials said. The buildings department will also examine all open and active crane rigging work performed by Bay Crane, the company operating the crane during Sunday's accident, as a precautionary measure, according to the mayor's office.

A representative from Bay Crane declined to comment Monday.

aConstruction professionals are to maintain and operate their crane equipment in a safe and code compliant manner at all times as required by the NYC Construction Codes. A failure to do so may result in the issuance of violations and additional enforcement action by the Department,a a building department spokesman said in an email. aIn addition to regularly scheduled inspections, the Department will continue conducting spot checks to ensure crane maintenance compliance.a

The building, 261 Madison Ave., remained evacuated Monday, and the buildings department was still at the scene. A sidewalk shed was erected across the street as a precaution at 260 Madison Ave. and one was also planned for the site of the accident, the building's department spokesman said.

Traffic surrounding the stretch of Madison Avenue was still closed off Monday afternoon, as the gray skies loomed overhead. It was expected to be reopened by Tuesday , according to the mayor's office.

The damage to multiple floors of the building, as well as metal sticking out from the side, was visible from the street below. Law enforcement officials were still monitoring the scene and keeping curious pedestrians at bay. About a dozen workers could be seen operating cranes, bulldozers, and putting up scaffolding around the building.

A representative for The Sapir Organization, who owns 261 Madison Ave., did not respond to a request for comment yesterday. The company said on Sunday that it was assessing the damages and working closely with city agencies.

(With Karina E. Cuevas)