One of the busiest bridges between the U.S. and Canada has been reopened to traffic after a bomb threat shut it down for hours. 

Detroit police spokeswoman Sgt. Eren Stephens said early Tuesday that the Ambassador Bridge was reopened at 1 a.m. ET 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, Ont.

Detroit police said a 911 call came in around 7:20 p.m. ET Monday to authorities on the U.S. side of the Ambassador Bridge. The caller said a bomb would go off in 10 minutes along the busy freight crossing, police Inspector Don Johnson said during a news conference Monday night. 

The call prompted authorities in both cities to halt all truck and car traffic across the bridge, Stephens said. 

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, though large trucks cannot use the underwater commuter tunnel. 

A similar threat was phoned in Thursday to Windsor authorities that lead 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. 

The Ambassador Bridge is North America’s busiest border and a commercial lifeline for many manufacturers in southwestern Ontario and the U.S. Midwest.