NEWARK, N.J. (AP) — A private security guard at Newark Liberty International Airport was arrested on charges of threatening Barack Obama the night before the president was to fly there.A Continental Airlines employee reported overhearing John Brek make threatening comments at an airport coffee cart Tuesday afternoon.The 55-year-old security guard was arrested several hours later, Port Authority spokesman John Kelly said. He denied making the threats, said Malcolm Wiley, a spokesman for the U.S. Secret Service.