Investigators began the grim task of evaluating the possible causes of the April 3 scaffold collapse onto a busy Boston street. A passing motorist and two workers were killed.

Workers for subcontractor Bostonian Masonry Corp., Norwood, Mass., and general contractor Macomber Builders, Boston, had been dismantling a mast-climbing platform lift that had been used to renovate a dormitory and student center building for Emerson College. When the accident occurred, sections of the mast-climbing platform lift fell onto Boylston Street onto a car driven by Michael Ty, 28. Two Bostonian employees, Robert Beane, 41, and Romildo Silva, 27, also were killed. Several other bystanders were injured and the accident closed the street for hours. The Occupational Safety and Health Administration and the Mass. Dept of Public Safety are investigating.

“The scaffolding and lifting device apparently ripped off the building, hit the roof of an adjacent building, then fell into the street,” says David M. Rosen, Emerson vice president for public affairs. Both the street outside the site, just off Boston Commons, and the college reopened April 4.

Officials of Macomber Builders, which held a press conference on April 3, could not be reached for comment on possible causes of the accident. OSHA officials declined to discuss whether Beane and Silva had been up on the building or on the street. The officials also declined to discuss how the two workers died.

The 20,000-ton platform has a 5-ton lifting capacity and is 48-ft long.