State and federal investigators began their probes of possible causes of an April 3 work platform 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 been used to build a dormitory and student center for Emerson College. When the accident occurred, sections of the mast-climbing platform lift fell, hitting a car driven by Michael Ty, 28, a physician. Two workers, Robert Beane, 41, and Romildo Silva, 27, also were killed. “The scaffolding and lifting device apparently ripped off the building, hit the roof of an adjacent building, then fell into the street,” says a spokesman for Emerson College.