A bomb threat concerning a United Airlines Holdings Inc. flight prompted a temporary halt to all flight operations at Ronald Reagan Washington National Airport on Tuesday. According to a person familiar with the situation, the ground stop was in effect until 1:30 p.m. local time, causing significant operational disruptions for the airline at a key hub. At the time of the report, it was not immediately clear how credible the threat was. United Airlines referred all inquiries regarding the incident to the Federal Bureau of Investigation. The temporary shutdown of a major airport due to a threat against one of its flights represents a serious security event with immediate, though localized, impact on the airline's services and passengers.