UPDATED: A bomb threat closed the tunnel between Windsor, Ontario, and Detroit for several hours Thursday, causing traffic backups as drivers on both sides of the border were rerouted,

reports CNN.

Around 12:30 p.m., employees who work for the Detroit-Windsor tunnel received an anonymous call from someone saying there was a bomb in the tunnel, Sgt. Matthew D'Asti with Windsor police told HLN. The call went to the Canadian side of the tunnel.

Authorities on both sides of the border investigated the tunnel for any possible device and were treating it "as a nonspecific, anonymous bomb threat," reports say.

Cars and buses were allowed back through the tunnel shortly after 4:30 p.m. Traffic from Canada resumed a few minutes later. (Cars make up the vast majority of the traffic through the tunnel.)

An estimated 27,000 to 29,000 vehicles pass through the tunnel every day. The tunnel is one of two international crossings between the cities of Windsor and Detroit. Traffic was being routed to the other crossing, the Ambassador Bridge, about 2 miles away.

Two groups battling over plans for a second bridge both tried to use the occasion to bolster t heir arguments over the need for additional border-crossing capacity.

Mickey Blashfield of the Detroit International Bridge Co. said later that the Ambassador Bridge doubled its normal traffic for the day. "Because overall border traffic remains down by more than 40% than a decade ago, capacity at the Ambassador Bridge accommodated all the traffic for the tunnel," he said in a statement. The Ambassador Bridge, privately owned, is opposing Gov. Rick Snyder's recent deal with Canada to build a second bridge.

On the other hand, at buildthedricnow.com, news stories were posted indicating that the closure illustrated what could go wrong if something happened to close the Ambassador Bridge. In a story from Michigan Radio, Lester Graham reports that congestion slowed traffic as vehicles were diverted from the tunnel.

"If the Ambassador Bridge were closed for hours or days for any reason, it would be a much different story," Graham writes. "Since the Ambassador Bridge is 83-years-old, the more likely scenario is some kind of repair which could restrict traffic or close the bridge, putting the economy in Michigan at risk."

Updated to include response from groups for and against a second border-crossing bridge.