The Eyewitness News Investigators' Jim Hoffer continues our coverage.
Crane accidents in the city are up by nearly 60 percent, and it now appears that more people died in Saturday's collapse than in all crane accidents in recent memory.
Getting to the bottom of what went wrong is critical if the city is to avoid future deaths.
Engineer David Peraza has 30 years of expertise investigating construction accidents. He says in the search for what went wrong on East 51st Street, it all comes down to the crane's collars, which brace the crane to the building.
"These collars and the attachment to the building are absolutely critical for the stability of the tower crane," he said.
On Saturday afternoon, all permits were in place to "install tower sections and collar ties" necessary to extend the crane higher.
But something went wrong. The nearly six-ton collar started sliding down the 18th floor, slamming into another collar on the ninth floor, slicing through the steel struts that held it to the building and plummeting further to another collar on the third floor. With the bracing severed, the 200-foot tower fell across an entire city block.
"Certainly, investigators will look at is whether one of the slings or other temporary supporting devices failed," Peraza said.
Still undetermined is whether it was human error or mechanical failure, though often times the distinction is unclear.
"There's a very fine line between human error and mechanical error when you're talking about a crane project," construction safety expert Jeffrey Manheimer said. "Even if it's a manufacturing defect, it should have been discovered by the contractor who was using it."
The last major crane collapse in the city occurred nearly a decade ago in Midtown, killing a worker and injuring three others. Since then, a building boom has increased the use of cranes across the city.
Some 250 are operating on any given day and only eight crane inspectors ensure they're run safely.
When they're not, and when something goes wrong, it usually turns out to have been preventable.
"Was there enough manpower on the jobsite?" Manheimer asked. "Were there enough taglines? Was there a competent person supervising that day at the jobsite?"
The Department of Buildings says it will be conducting a city-wide inspection of cranes as a precautionary measure. Also, the building trades union, which represent 100,000 construction workers, has already begun immediate re-inspections of crane operations.