Witnesses described hearing a rumble, then the crash of scaffolding that kicked up metal, dust and boards on Boylston Street, which runs along the south side of Boston Common.
The platform lift system and scaffolding were set up atop a new building next to a 14-story dormitory being constructed by Emerson College. They had been used to install a stone facade and were being dismantled when the accident occurred, said acting Fire Commissioner Kevin MacCurtain.
John D. Macomber, president and CEO of Macomber Builders, the lead contractor hired by Emerson, said at a news conference that the dead were believed to be a passer-by and two workers for subcontractor Bostonian Masonry.
Two people were injured, said fire Lt. David M. Pfeil.
The company is still investigating, Macomber said.
"It looks as though one of the moving, movable scaffoldings fell off the side of the building out toward Boylston Street and down. We don't know why that happened yet. They're tied in laterally very well," Macomber said.
It was a scary scene for those nearby.
"I had just walked through the spot where it fell when I heard this roar," said Dan Rofsky, 19, an Emerson freshman from New Jersey. "I just saw this cloud of dust and smoke."
No students were injured, said David Rosen, Emerson's vice president of public affairs. The building — a dormitory and campus center — has been under construction for nearly two years and was scheduled to open for the fall semester, he said.
Brian Trimmer, assistant manager of nearby Commonwealth Books, said he looked out the store's front window just as an eight- to 10-foot chunk of machinery crashed onto a midsize silver Honda.
"I saw this large orange thing fall out of the sky on top of a car," Trimmer said. "It flattened it."
John Hynes was driving his BMW in the opposite direction of the Honda when the platform and scaffolding started to tumble.
"You could see it coming down and then I started rolling forward. It started to hit my car, and then I sped up," said Hynes, a Boston resident and grandson of former Mayor John Hynes. He was not injured.
Macomber, based in south Boston, has been in business for more than 100 years and helped build Boston's Faneuil Hall Marketplace, according to the company's Web site.