Another fake bomb threat came in yesterday concerning the  Ambassador Bridge.  A call Monday led to the closing of the crossing for several hours, but this time authorities kept the bridge open while they investigated the call.  Police say local and U.S. authorities “officially cleared” the bridge at 7 p.m.  On July 12, a telephoned bomb threat closed the Detroit-Windsor Tunnel for some four hours, and Tuesday of this week  Comerica Park remained despite a bomb threat during a Detroit Tigers game.  In each case, no bombs were found.  As of yesterday, authorities in Detroit continue to determine if the Comerica Park threat was linked to the threats at the bridge and tunnel. No arrests have been made.