A bomb threat closed the Detroit-Windsor Tunnel for nearly four hours Thursday, but officers from police departments on both sides of the border found no explosives inside the 1.5-kilometre international crossing.

Canada Border Services Agency officers were permitted to return to their posts just after 4:30 p.m. ET. The border reopened by 5:05 p.m., and as of 6 o'clock there was no delay for those entering Canada and a five-minute wait to enter the United States.

The threat was called in between 12:30 and 1 p.m. to the Tunnel Duty Free Shop near the tunnel's Windsor-side entrance, said Carolyn Brown, executive vice-president of the tunnel corporation.

The caller simply said, "There's a bomb in the tunnel," police spokesman Sgt. Matt D'Asti said.

"We have nothing to go on other than a phone call," D'Asti said. "It was a brief phone call."

He confirmed police dusted a payphone for fingerprints near the tunnel entrance on the Windsor side.

"At this stage, there is indication [the call] may have been from a [phone] booth," D'Asti said.

Police dusted this payphone for fingerprints. It's across the street from the entrance of the Detroit-Windsor Tunnel on the Canadian side. (CBC)

The tunnel was evacuated in both directions. Travellers who had already driven through but not made it past customs were expedited by border agents on both sides.

The tunnel, which connects downtown Windsor with Detroit, can handle as many as 2,000 cars an hour. It is used more by passenger traffic, tourists and businesspeople than for trade. The nearby Ambassador Bridge is North America's busiest border crossing.

Police from both sides of the border, including the Windsor and Detroit bomb squads, engaged in the search, accompanied by tunnel employees. The Detroit police bomb squad's spokesman, Don Johnson, said officers used bomb-sniffing dogs to look for any explosives.

Windsor police Deputy Chief Jerome Brannigan added that officers were examining video from inside the tunnel.

River access closed

At one point, around 1:30 p.m., an empty flat-bed truck went into the tunnel and exited carrying an Oldsmobile. Police later said the car was not related to the threat and was just a broken-down vehicle.

During their investigation, law enforcement personnel turned pleasure boats away and stopped freighters on the Detroit River. Boats were met on the water west of the Ambassador Bridge and east of Caesars Windsor.

Transit Windsor operations manager Pat Delmore said two public-transit buses from Windsor were in Detroit, as part of their regular cross-border route, at the time of the threat. One returned via the Ambassador Bridge. The other was scheduled to return at around 4 p.m.

Additional buses were sent to the U.S. via the bridge and returned with commuters.

This isn't the first time a threat has closed the international crossing. "In the wake of 9/11, we did receive a couple of threats and we had to close, but not for this length of time," Brown said.

She said staff practise for such threats on an annual basis.