Emergency responders work Wednesday at a former power plant building in South Boston after a partial collapse.
CNN  — 

A construction worker whose legs got trapped for more than three hours after a floor at a former power plant in South Boston collapsed has sustained life-threatening injuries, Fire Commissioner Jack Dempsey said at a news conference Wednesday.

Two other workers had minor injuries and were taken from the scene immediately, Dempsey said.

Dempsey said a large piece of cantilevered flooring fell, forming a void and trapping the worker’s lower legs.

About 100 emergency responders helped in the rescue, including EMTs and paramedics who helped the victim with pain management. A surgeon was also brought to the scene due to the seriousness of the victim’s injuries.

The collapse at the former Edison Power Plant occurred around 1:40 p.m. and the worker was extricated just after 5 p.m. The collapse was the second one at a Boston construction site in the past few months. In March, a construction worker was killed in a partial collapse at a Boston parking garage that was being demolished.

Boston Mayor Michelle Wu on Wednesday thanked emergency responders.

“It is a near miracle that our third person was able to be transported to the hospital safely, and we are sending every prayer for a quick stabilization and recovery,” the mayor said.

Dempsey said the building is old and weak, and “demolition jobs are dangerous.”

The worker who was trapped was conscious throughout the rescue, he said.

According to CNN affiliate WCVB, the plant is not active and the site was sold to a developer who has proposed a mixed-use development with residential, retail and hotel space.