ATLANTA, Jan. 24 (UPI) — Two airplanes at the Hartsfield–Jackson Atlanta International Airport, from Delta Airlines and Southwest Airlines, are being searched after a bomb threat on Saturday.

Stephen Emmett, a spokesman for the FBI, told the New York Times the threats were credible. Two F-16 fighter jets were scrambled to escort the planes to the airport.

Parts of the Atlanta International Airport have been shut down while the threat is assessed.

The bomb threat was received via Twitter, and Southwest says “customers and the aircraft are being rescreened.”

The airport is the world’s busiest by passenger traffic and by number of landings and take-offs.

UPDATE: No bombs have been found on the planes, and the FBI will be searching for the person who made the threat.