CHICAGO -- Scaffolding from the landmark John Hancock Building fell on a busy downtown street and crushed cars Saturday, killing three people and critically injuring two, a fire department spokesman said.

Fire officials said high winds may have caused a 25-foot section of scaffolding to fall from the 43rd floor. Gusts of up to 58 mph were recorded at a downtown airport around the time of the collapse, just before 2 p.m., the National Weather Service said.

Krista Galaida, who was working in a flower shop on the first floor of the Hancock building, said she heard what sounded like an explosion.

"We just bolted; it was so loud and the ground shook," said Galaida. "We thought a truck had come into the building or something. Everybody thought it was a bomb."

Shaylan Baldwin saw the scaffolding hit the cars as he was leaving his apartment.

"We just started running down there and we got right up to the cars to see if there was anyone in there. It was totally crushed, it was up just to my waist," Baldwin said.

Officials said they believe the scaffolding was on the west side of the building, on Michigan Avenue, then swung away before falling onto three cars waiting for a stop light on the south side of the building. Scaffolding also shattered dozens of windows in the Hancock building, sending glass raining down on the streets below, which are typically bustling on the weekend with thousands of residents and tourists,

Scaffolding also shattered a window in the Westin Hotel lobby, north of the Hancock building, officials and witnesses said.

The Medical Examiner's office would not immediately release the names of the deceased Saturday night.

One woman was in critical condition and two were in good condition at Northwestern Memorial Hospital, administrator Shelley Williams said but did not have details about their injuries and declined to release the victims' names.

A 56-year-old woman was in critical condition with injuries that included a broken femur and another was in fair condition at Cook County Hospital, a hospital administrator said. A 56-year-old man was treated and released from Grant Hospital for an injury to his right shoulder caused when he tried to help remove someone from a car, a nursing supervisor said.

Streets for two to three blocks around the building were closed and the building evacuated below the 42nd floor, as pieces of scaffolding continued to dangle from the building and wind whipped through broken windows, officials said.