The Detroit Windsor Tunnel was closed to traffic after the threat was called in on the Canadian side, tunnel chief executive Neal Belitsky told The Associated Press. The call was made some time after 12:30 p.m. to the duty free shop on a plaza on the tunnel's Windsor side, tunnel executive vice president Carolyn Brown said.
The underwater tunnel stretches about a mile beneath the Detroit River, which is one of North America's busiest trade crossings.
Initially, traffic going onto plazas on both sides of the tunnel was halted and diverted away from the tunnel. Officials then decided to clear the tunnel and block off entrances and exits on both sides, Brown said.
Bomb squads from the Detroit and Windsor police departments were called in to help with the investigation. Paramedics and a Detroit Police tactical unit truck was on scene on the Detroit side. Jefferson Avenue through downtown was closed.
Commuter rush-hour traffic on both sides was redirected to the Ambassador Bridge, which is just west of both downtowns. Cars and buses make up most of the traffic along the 82-year-old tunnel, which was used by about 4.5 million cars in 2011.
"As security concerns were made, customs became a little more thorough at the bridge" said Ambassador Bridge spokesman Mickey Blashfield. "Traffic is up to speed coming into the United States. There is a delay going into Canada. That's a function of inspections, not border capacity."
Brown said the security steps appeared to be running smoothly.
"We practice this," she said. "Once a year we do a full-blown exercise. We shut it down on a Sunday morning and we have all the first responders in. We simulate an accident or an incident."