Boston ? A 10-ton construction platform collapsed and crashed 13 stories onto a busy downtown street Monday, killing three people and crushing cars stopped in midday traffic.

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.