DETROIT (WKZO) -- For the second time in less than a week, officials are investigating a bomb threat at an international crossing between Detroit and Windsor, Ontario.  Police say an anonymous caller from Detroit claimed a bomb would explode on the Ambassador Bridge Monday evening, prompting authorities to close the crossing in both directions along with a stretch of the Detroit River.  Last Thursday, a bomb threat originating in Canada closed the Detroit Windsor tunnel for several hours before authorities determined the call to be a hoax.