One construction worker was killed and a second was critically injured in Cambridge on Wednesday morning when a section of the concrete stairwell in a city-owned parking garage collapsed, authorities said.

“At least a ton” of steel and concrete fell during the collapse of the second-story stairwell, which was undergoing repairs, acting Fire Chief Gerard Mahoney said during a news conference.

No names were released but the man who died at the scene was 56, while a 41-year-old man was taken to the hospital with critical injuries, according to a statement from the Middlesex district attorney. Mahoney described it as a chest injury.

No one else was in the stairwell at the time, but the garage has remained open during the work.

The fire department reported that it responded to the scene at about 8:30 a.m. for a “structure building collapse.”

Both men were employed by a subcontractor.

State and local authorities as well as the federal Occupational and Safety Health Administration are investigating.

The accident occurred a week after two construction workers were killed in downtown Boston when they were struck by a truck during sewer work.

It was not immediately clear which project the men were working on, however work on part of the garage is involved in Leggat McCall Properties’ revamp of the former Sullivan Courthouse nearby.