July 17, 2012 -- Updated 0228 GMT (1028 HKT) The Coast Guard closed the Ambassador Bridge between Michigan and Ontario late Monday in response to a threat. The U.S. Coast Guard closes the Ambassador Bridge between Detroit and Windsor, Ontario The action comes after the Detroit Police Department received a threat Authorities are investigating, and maritime traffic is stopped A Coast Guard spokesman says it is "too soon" to determine who sent the threat (CNN) -- A major bridge between Detroit, Michigan, and Windsor, Ontario, was shut down late Monday due to a bomb threat, the U.S. Coast Guard said. The Ambassador Bridge was secured and cleared on both sides after the Detroit Police Department received the threat at 8 p.m., according to U.S. Coast Guard Capt. Jeff Ogden. The U.S. Coast Guard stopped all maritime traffic below the bridge. Local police agencies were investigating the threat, but Ogden said it was "too soon" to determine who sent the threat. "We take all threats seriously," he said, "especially along the border." The Ambassador Bridge is the busiest commercial international crossing in North America with more than 10,000 commercial vehicles crossing the bridge on a typical weekday, according to the bridge's official website. More than 25% of all commercial goods between Canada and the United States cross the Ambassador Bridge, the website said. The closing Monday night followed last Thursday's closing of the Detroit-Windsor Tunnel, which also was prompted by a bomb threat.