Chicago ? Scaffolding from the landmark John Hancock Center fell more than 40 stories onto a busy downtown street and crushed cars Saturday, killing three people and injuring at least seven.

High winds may have caused a 25-foot section of scaffolding to fall from the 43rd floor, fire officials said. Gusts of 58 mph were recorded at a downtown airport around the time of the midafternoon collapse, 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,” Galaida said. “We thought a truck had come into the building or something. Everybody thought it was a bomb.”

The scaffolding, erected on the west or Michigan Avenue side of the building, swung away and fell onto cars stopped at a traffic light south of the building. Dozens of windows shattered in the Hancock building, raining glass onto streets typically bustling with thousands of residents and tourists.

The broken scaffolding was tied to the building later Saturday as wind whipped through broken windows. The building was evacuated below the 42nd floor, and streets were closed because of worries of falling debris.

A window in the Westin Hotel lobby, north of the Hancock building also was shattered.

The scaffolding had been erected so crews, who weren’t believed to be working Saturday, could repair and clean the face of the building, Fire Commissioner James Joyce said.

Those killed were in three cars crushed by the debris, Joyce said.

One woman was in critical condition at Northwestern Memorial Hospital, administrator Shelley Williams said. Four others were treated and released.

A 56-year-old woman was in critical condition with a broken leg and other injuries and another person was in fair condition at Cook County Hospital, a hospital administrator said.

A man was treated at Grant Hospital for a shoulder injury caused when he tried to help remove someone from a car, a nursing supervisor said.

The 100-story Hancock building, the city’s third-tallest building, anchors the north end of the city’s busy Michigan Avenue shopping district.

Police spokesman Pat Camden said the streets would remain closed until the scaffolding could be removed, but he did not know when that would be completed.

Joyce said the accident “should serve as a warning,” to owners of other buildings with scaffolding.

“This is a dangerous wind,” Joyce said.