New York - Bomb threats against two US airline flights on Saturday prompted North American air defence fighter planes to scramble to accompany them to their destination in Atlanta, an airport spokesperson said.The planes landed safely at Hartsfield-Jackson International Airport, no bombs were found aboard either and the Atlanta airport has returned to normal operations, said airport spokesperson Reese McCranie.Threats against Delta Air Lines flight 1156, coming from Portland, Oregon, and Southwest Airlines flight 2492, from Milwaukee, had been deemed credible, McCranie said.Several media outlets reported that a Twitter user going by the name Zortic wrote of planting bombs on the two planes, stating in a message to a Delta Twitter account, "I have a bomb on one of your planes, but I forgot which one when I left the airport. Can you help me find it?"Those tweets have since been deleted.Neither the Federal Bureau of Investigation, which is leading the investigation, nor the airport confirmed the source of the threats.The North American Aerospace Defence Command scrambled fighter jets from McEntire Joint National Guard Base in South Carolina, said NORAD spokesperson Preston Schlachter. A pair of fighter jets accompanied each aircraft as it landed, he said.Southwest said in a statement 86 passengers were aboard its flight, and they were being rescreened.A Delta spokesperson said by email its plane was carrying 180 passengers.The incident comes five days after bomb threats were made against two other Delta flights. One was arriving at John F Kennedy International Airport in New York from San Francisco, and another was departing the New York airport for Tel Aviv.