The March 15 crane collapse that killed seven people in New York City is being blamed on a broken nylon sling. The New York Times has a story, and photos, posted on its Web site today.

Investigators believe the accident occurred as workers were trying to install a massive square steel collar around the crane’s tower, at the 18th floor of the construction site. They were using a series of manual winches that appeared to have been hung from nylon slings attached to a higher portion of the tower. The collar was to have been attached to the building by steel struts to give the tower added stability.

But the collar broke free and — along with the winches and slings — plummeted down the outside of the shaft, smashing into a second collar at the ninth floor and shearing it from the building before coming to rest on top of a third collar near the base. That destabilized the tower, and the weight of the crane’s cab pulled the tower down onto the buildings to the south, damaging several on 51st and 50th Streets and completely demolishing the town house where the bodies were recovered on Monday.

For investigators who arrived at the site after the accident, the ragged, broken slings immediately raised alarms, according to people involved in the recovery. Construction safety experts said the slings typically cost about $50 and, depending on their size, can lift moderate loads or loads of several tons. But they warned that if the slings are worn or damaged, their strength may be greatly reduced.

The story quotes Paul Zorich, chairman of the committee on crane and sling safety standards of the American Society of Mechanical Engineers, who examined the photos and believes the sling may have been "grossly overloaded.”

Another expert, Steven Dewey, president of All-Lifts, a company in Albany, N.Y., that manufactures construction slings, said slings generally fail only when they are cut or damaged.