Authorities have received more than 50 threats directed at U.S. airlines since Jan. 17 when a flight between Atlanta and Raleigh received a bomb threat, a U.S. official said. At least 20 of those have been recorded since Saturday on Twitter.

"We are continuing to investigate these threats with our law enforcement and airline partners as we do with all stated threats," an FBI spokesman told CNN on Wednesday. "Threats of this nature can and do result in costly responses from a multitude of law enforcement and airport entities and greatly inconvenience travelers. Individuals responsible can be prosecuted federally."

Three separate Twitter accounts have been identified as issuing at least eight of the threats. One account made threats against six separate planes: an American Airlines flight from San Francisco to Chicago, a Southwest Airlines flight from Chicago to Charlotte and four different Delta flights.

A second account targeted an American Airlines flight from Los Angeles to Chicago on Jan. 28. The tweet read, "@AmericanAir No, There is a bomb on Flight 1192, We Are ISIS, we will (expletive) you guys up, #ISIS"

A third account described a bomb and passenger with a gun aboard a United Airlines flight between Newark, New Jersey, and Miami. The flight was scheduled for Tuesday afternoon but was canceled during the blizzard that hit the northeast on Monday night and Tuesday.  

On Jan. 19, a Delta Airlines flight was evacuated at John F. Kennedy International Airport after receiving a bomb threat. On Saturday, fighter jets escorted two planes to Atlanta’s airport after bomb threats were published on Twitter. The following day, a Delta Airlines flight from Los Angeles to Orlando, Florida, was diverted to Dallas/Fort Worth International Airport for the same reason, NBC News reports

All security threats made to planes that are in the U.S. or heading to the U.S. must be reported to the Transportation Security Administration. However, the definition of what is considered a threat is vague in the guidelines to prevent airlines from underreporting, according to a Government Accountability Office report in 2007. An airline’s security division is responsible for assessing the threat and suggesting a course of action. A flight can be diverted to the nearest airport, evacuated and searched with bomb-sniffing dogs.

“We're seeing these new threats,” Glen Winn, former head of security at Northwest Airlines and United Airlines and an instructor at the University of Southern California School of Aviation Safety and Security, told USA Today on Wednesday. “In terms of the quantity of (online) threats we're seeing now, you just haven't seen it.”