More bomb threats continue to hound flights bound for the U.S., with the latest made on Delta Air Lines Flight 1156 and Southwest Airlines Flight 2492.

Reese McCranie, spokesman of the Hartsfield-Jackson International Airport in Atlanta said authorities found no bomb in the Delta Air Lines jet that flew in on Saturday from Portland, Oregon. However, the Southwest Airlines was still being searched by explosives units of the FBI, said Special Agent Stephen Emmet, spokesman of the FBI.

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New York Daily News eventually said that the Southwest plane, which arrived at 2 p.m. was also cleared of bombs. The Delta jet arrived earlier at 1:30 p.m.

The bomb threats were made on Twitter, but investigating agencies said the source of the threats were unknown. To ensure the safety of the passengers, the North American Aerospace Defense Command deployed two fighter planes to accompany the two aircraft until it landed in the Atlanta Airport, Reuters reports.

The tweets were made by someone using the handle @KingZortic, which were discovered by a Delta employee before 1 p.m. on Saturday. The tweet reads: "I have a bomb on one of your planes, but I forget which one when I left the airport. Can you help me find it?"

Emmet said the threat appears credible. @KingZortic said the bomb was smuggled aboard the Delta aircraft due to weak security measures at the back of the plane and in connivance with a Delta staff.

Travelers from the two planes were ordered to disembark from their planes. Southwest rescreened the 86 passengers aboard the flight from Milwaukee.

Both police bomb and K-9 units searched the two planes while grounded at the airport's tarmac.

Tagsbomb threat, Delta Airlines, Southwest Airlines, atlanta