Emergency workers resumed a slow and delicate search yesterday for the bodies of three people believed to be buried in rubble after a crane at an Upper East Side high-rise construction site collapsed over the weekend.

Four construction workers were already confirmed dead this weekend in what the mayor called one of the city’s worst construction accidents in recent memory. The three missing include two construction workers and a woman that a police source said was sleeping inside an East 50th Street townhouse on Saturday afternoon when the crane fell off a nearby building, careered across two city blocks, and crashed through the roof.

Mayor Bloomberg described the woman as the girlfriend of the townhouse’s owner, and the police source said it was believed she was visiting from Florida.

City officials said they have not ruled out the possibility that the three missing people might still be alive. But hopes dimmed throughout the day as firefighters and construction workers picking through the precarious ruins were slowed by heavy winds and the dangers of the task.

Listening devices were being inserted into holes in the rubble as debris was removed to check for signs of life, but at an afternoon news conference Mr. Bloomberg said, “We’ve heard nothing.”

More details emerged yesterday about the operation that led to the crane’s collapse, although authorities said it could take days or weeks to determine whether the cause was a mechanical failure or human error.

The Department of Buildings commissioner, Patricia Lancaster, said construction workers building a 43-story high-rise at 303 E. 51st St. were extending the crane, a routine operation in which pieces of crane are added to the top and then bolted in by hand. Workers on Saturday also added a new collar to the crane; the collar links the crane to the building with steel beams to keep the crane from toppling over.

Ms. Lancaster said the collapse occurred after the new collar at the 18th floor gave way and dropped, shearing through another collar that attached the crane to the building at the ninth floor.

There was an additional collar on the third floor, but after the top two collars broke away from the building, Ms. Lancaster said the crane was too top heavy to stay upright.

“It could have been something misaligned or something. It could have been that the straps holding it didn’t hold. It could have been that somebody jumped on it by mistake. The weight is the thing that causes things to fail,” she said. The crane pulled southward, hitting a 19-story apartment building across the street and splitting apart. A 75-foot piece of the crane continued to slide until it fell off the roof of the 19-story building and landed on top of the four-story townhouse below.

About 250 tower cranes are now operating in the city and they are rarely involved in accidents, Mr. Bloomberg said yesterday.

The crane in Saturday’s accident is manufactured by Favelle Favco, which made a crane that caused a fatal accident a decade ago in Sydney, Australia. According to reports in Australian newspapers, Favco was not likely to be charged in that accident.

A Favco crane also was involved in an accident in Manhattan in 1990 that left a pedestrian injured. Favco’s American branch office could not be reached for comment yesterday.

Residents from 12 nearby buildings who were evacuated were blocked from returning home yesterday afternoon, and four streets, including 10 blocks of Second Avenue, were closed to traffic. The mayor said the city would work to have one lane of Second Avenue opened to morning rush hour traffic.

Local elected officials, including the president of Manhattan, Scott Stringer, and Rep. Carolyn Maloney, held a news conference yesterday near the site of the accident to call on the city to increase its supervision of construction sites.

“We cannot allow one more death, one more injury,” Mr. Stringer said, calling a recent spate of fatal accidents around the city a “construction crisis.”