One of the busiest bridges between the US and Canada has been reopened to traffic after a security threat shut it down for hours, just four days after a similar threat closed a nearby commuter tunnel that also connects the US city of Detroit to Windsor, Ontario.Eren Stephens, Detroit police spokesperson, said early Tuesday that the bridge had been reopened at midnight after security sweeps failed to turn up any incendiary devices.Detroit police said a call came in around 7:20pm to authorities on the US side of the Ambassador Bridge. The caller warned that a bomb would go off in 10 minutes along the busy freight crossing, police Inspector Don Johnson said during news conference Monday night.The call prompted authorities in both cities to halt all truck and car traffic across the bridge.Police had set up several roadblocks near the bridge, where canine units were called in to search the bridge. The Coast Guard was patrolling the Detroit River beneath the bridge and blocked river traffic.Surveillance videoThe closing led to major traffic backups on expressways and other major roads leading into central Detroit, Stephens said. Car and light truck traffic was being diverted to the Detroit Windsor Tunnel about 3km away, though large trucks cannot use the underwater commuter tunnel.