Video, computer model to aid in bridge investigation.

Byline: Jon Hilkevitch

MINNEAPOLIS _ The emergence Thursday of a dramatic security video capturing the catastrophic collapse of Minnesota's busiest bridge, and help from a computerized model of the span, ultimately "will tell us where the failure began," the leader of the accident investigation said.

Still, it likely will take months to pinpoint hidden defects in the bridge that may have gone undetected for years, officials said, and a year or longer to identify the probable cause of the disaster.

The chilling video, captured by a security camera at an industrial site, shows the Interstate Highway 35W bridge breaking apart under tremendous forces and crashing into the Mississippi River during Wednesday evening's rush period.

It took only four seconds for the massive steel-arched bridge to buckle and fall.

Dozens of vehicles plummeted 64 feet into the Mississippi when the eight-lane bridge gave out. Divers resumed the dangerous search Thursday for bodies trapped under tons of twisted debris. The official death toll stood at four victims, but up to 30 people remained missing, officials said. About 80 survivors were treated for…