Firefighters respond Wednesday to a stairwell collapse at the city-owned First Street Garage. (Photo: Cambridge Police Department)

A collapsing stairwell at the city’s First Street Garage in East Cambridge killed a 56-year-old construction worker and left another man with “significant” chest and arm injuries early Wednesday, acting fire chief Gerry Mahoney said. The worker, 41, was taken quickly to a local hospital and is in critical condition.

Christopher Stuck, 56, of South Windsor, Connecticut, was declared dead on the scene. He was identified Thursday by Middlesex District Attorney Marian Ryan, Mahoney and Cambridge police commissioner Branville G. Bard Jr.

The victims were doing restoration work on the second floor of the 55 First St. garage, across from the CambridgeSide mall, when a stairwell collapsed shortly before 8:23 a.m., when a 911 call about the collapse came in, Mahoney said during a press conference at the scene around an hour and a half later. They were working in the stairwell as it collapsed, he said.

In a structure made of steel risers with poured concrete on top, it was “probably at least a ton” of material that collapsed, Mahoney said.

“Upon arrival, fire companies discovered a partial interior collapse of a stairway that was being disassembled because the stairway was being replaced,” Mahoney said. “We are awaiting the arrival of some structural engineering folks, as well as representatives of [the Occupational Safety and Health Administration] … we’re waiting for a plan to be put in place after consultation with engineers on how we’re going to proceed to render the rest of the scene safe.”

He did not have the names of the victims, who were working alone in the garage for a subcontractor of a company Mahoney identified as Structural Engineering Preservation of Woburn. They were removing the closed stairwell in an otherwise in-use parking garage because it was going to be replaced, he said.

It was not known who called 911.

The collapse was called unusual. “In a city like Cambridge, I’ll have 38 years on Saturday [as a Cambridge firefighter] and I’ve been to three or four of these,” Mahoney said. “it’s part of the job, but it’s something you really deep down don’t get used to. But that’s the business.”

This post was updated March 4, 2021, with the name of the man killed on the scene.