Two hurt in building collapse at Boston construction site

Fire officials said at the scene, which is on the edge of the Chinatown district, that a “dead load” fell from the 12th floor, the highest level the project had reached so far, to the fifth floor. Boston Fire Department

Three men were injured Thursday when a roof collapsed inside a 33-story building under construction at the edge of Chinatown and Downtown Crossing, causing a “pancake” reaction with floors falling atop one another, the Boston Fire Department said.

Two of the victims were taken to nearby Tufts Medical Center. Their injuries were not considered life-threatening. One had minor injuries; the other had a serious head wound, said Fire Department spokesman Steve MacDonald.

A third man hurt his wrist and walked to the hospital for treatment, MacDonald said.

The accident happened about 8 a.m. when 120 iron workers, laborers, and carpenters were inside the structure at 45 Stuart St., MacDonald said.

“The roof collapsed from the 12th floor to the fifth,’’ said MacDonald. “It did what we call pancake,’’ with one floor flattening the other below. He said he was unsure on which floor the injured laborers were working. He said inspectors from the Occupational Safety and Health Administration, the city, and the state were examining the building. OSHA will investigate the incident, he said.

At a morning event Thursday, Mayor Martin J. Walsh expressed concern for the laborers. “There are a lot of construction workers, men and women, in the trade very shaken up over this,” the mayor said.

The owner of the building was identified as AvalonBay Communities Inc., and an AvalonBay representative said the construction company is John Moriarty & Associates.

The building, a residential tower, was expected to be completed by mid-2015.

At the site early Thursday, a massive crane towered over the building an hour after the incident. Two Fire Department aerial ladders extended up the side of the building on the LaGrange Street side.

Michael Roberts, senior vice president of development for AvalonBay, said the company was hopeful the injured workers “will make full recoveries.’’

“We are fully cooperating with the Fire Department investigation and are in contact with representatives of OSHA,’’ Roberts said.

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Martin Finucane of the Globe staff and Globe correspondent Catalina Gaitan contributed to this report.