(NBC News) - At least five bomb threats were phoned in Tuesday against flights originating or landing in the United States, government sources told NBC News. The sources said that the threats were not deemed credible.
Four of the five flights — one each from US Airways, Delta Air Lines, United Airlines and the Mexican carrier Volaris — landed. The fifth, Korean Air Flight 23 from Seoul to San Francisco, was still in the air, scheduled to land Tuesday afternoon.
In Philadelphia, police met the US Airways plane, Flight 648 from San Diego, when it landed. NBC Philadelphia reported that a police bomb squad, including dogs, searched the plane and passengers and gave the all-clear.
The three other flights that had landed were Delta Flight 55, from Los Angeles to Atlanta; United Flight 995, from San Francisco to Chicago O'Hare; and Volaris Flight 939, from Portland, Oregon,to Guadalajara, Mexico.
Hoax threats have been made against almost a dozen planes over the past two weeks, mostly flights coming into the United States from other countries.