Detroit police spokeswoman Sgt. Eren Stephens said early Tuesday that the Ambassador Bridge was reopened at 1 a.m. after security sweeps failed to turn up any incendiary devices.
She said traffic was backed up but is expected to be clear by Tuesday morning's rush hour.
"We have over 100 trucks (waiting to cross), so it's going to take some time," she said.
Monday's bomb threat came just four days after a similar threat closed a nearby commuter tunnel that also connects Detroit to Windsor, Ontario.
Detroit police said a 911 call came in around 7:20 p.m. Monday to authorities on the U.S. side of the bridge. The caller said a bomb would go off in 10 minutes along the busy freight crossing, police Insp. 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, Stephens said.
There were no immediate reports of authorities finding a bomb, though helicopters continued to fly overhead late Monday night. Police had set up several roadblocks near the bridge, where canine units were called in to search the bridge. The U.S. Coast Guard was patrolling the Detroit River beneath the bridge and blocked river traffic.
The closing led to major traffic backups on expressways and other major roads leading into downtown Detroit, Stephens said. Car and light truck traffic was being diverted to the Detroit Windsor Tunnel about three kilometres away, though large trucks cannot use the underwater commuter tunnel.
A similar threat was phoned in Thursday to Windsor authorities that led to a four-hour closing of the Detroit Windsor tunnel, a busy border crossing beneath the river that also connects the two border cities. No explosives were found.
At the time, police said a man had called from a street pay phone and warned of a bomb on the Canadian side of the tunnel. Authorities have since said surveillance video from a coffee shop near the phone booth where the call was made might offer clues about the caller.
Stephens said Detroit police are leading the investigation into the bridge threat and working with state and federal law enforcement agencies.
Johnson said the call came in from somewhere in Detroit, though other details weren't immediately released.
Dan Stamper, president of the company that owns the bridge, told WJBK-TV that crews thoroughly inspected the bridge and had found no signs of explosives by Monday night.