More than 20 U.S. embassies in 17 countries closed Sunday amid a serious terror threat. Richard Engel, NBC's chief foreign correspondent, said that the threat was specific in timing and location and focusing on Yemen. The warning was later expanded as a safety measure, Engel said."Terrorists may elect to use a variety of means and weapons and target both official and private interests," the State Department said in its first travel alert since Osama bin Laden was killed. The alert will remain in place through the month of August.General Martin Dempsey, chairman of the Joint Chiefs of Staff said on ABC's This Week that the threat was made to western interests, not solely U.S. concerns.NBC's Kristen Welker and Richard Engel reported more on Weekends with Alex Witt.