Updated 12/29/14 – 2:38 p.m.

CHICAGO (CBS) — Four construction workers were injured Monday morning in an accident at a building site along the Chicago River, just west of downtown.

Assistant Deputy Fire Commissioner Mark Nielsen said, shortly after 9 a.m., a steel I-beam collapsed at a construction site for River Point, a 52-story skyscraper at the northeast corner of Lake and Canal streets, on the west bank of the Chicago River.

Officials with the U.S. Occupational Safety and Health Administration said three ironworkers were erecting a steel beam, using a standard two-bolt connection system, when two bolts on the beam on which they were standing gave way, and the beam fell 14 to 20 feet. The employees had been secured to the beam, as required, and a safety net had been placed under the beam.

A fourth worker on the ground was struck by debris from the collapse.

The three workers who fell ended up several feet below street level, and firefighters had to lift them out to get them to ambulances.

“The last victim took a little bit of time. He was below grade, and when the rescue squad company got here, they had to package him properly. They provided ALS [advanced life support] care, started an IV, and took good care of him, and they took him out in a Stokes basket,” Nielsen said.

All four workers were conscious when they were taken to hospitals — three of them in serious condition.

OSHA said it appeared two bolts sheared off the beam, causing it to fall.

OSHA investigators were at the site Monday afternoon, and have cordoned off the area where the accident occurred, and halted construction in that area while they look into the accident. Work was allowed to continue at other parts of the construction site.

It was unclear how long OSHA’s investigation would take, but the agency has six months to complete its probe.

Firefighters respond to a construction accident at Lake and Canal streets, after a steel beam collapsed, injuring four people. (Credit: Chicago Fire Department)

Firefighters respond to a construction accident at Lake and Canal streets, after a steel beam collapsed, injuring four people. (Credit: Chicago Fire Department)