Four iron workers were injured at approximately 9:05 a.m. Monday when a steel I-beam collapsed at the site of a 52-story office tower under construction in Chicago's West Loop.

Though the circumstances are unclear, “Two workers were actually on the part that collapsed about 20 feet above grade and then they fell into a hole 10 feet below grade,” Assistant Deputy Fire Commissioner Mark Neilsen told reporters this morning. “There might have been a couple people below when the collapse occurred..

Media accounts indicate three of the workers were transported to local hospitals, where they were listed in “serious to critical” condition. The fourth worker reportedly was listed in “fair-to-serious” condition. All four are employed by Detroit-based contractor Midwest Steel Inc.

Members of the Occupational Safety and Health Administration were called to the site shortly after the accident occurred. OSHA spokesman Scott Allen indicated the accident is under investigation.

Work on the project, a joint-venture between Canadian developer Ivanhoe Cambridge and Hines Interests, was suspended, pending further investigation. Lend-Lease and Clark Construction Group, both of Chicago, are general contractors for the project, which broke ground in January 2013. The project, named River Point, was the largest commercial structure to break ground in five years.