Two planes have been searched at New York's Kennedy Airport after threats were made against the flights.
The FBI says US military jets scrambled to escort an Air France flight into the city after an anonymous caller claimed a chemical weapon was aboard the aircraft. The airliner landed safely and a search found no weapons.
They say a Saudi Arabian Airlines flight was also being checked out on Monday because of a threat.
The FBI believes the same caller made the threats and the airliners were being screened as a precaution.
A Saudi Arabian Airlines flight arriving at Kennedy was also being checked out because of another threat, authorities said.
A third threat was made against an American Airlines jet flying from Birmingham, in the UK, to Kennedy while it was still in the air, said airline spokesman Kent Powell.
Authorities initially told the pilot to land and taxi to a remote area away from the terminal but later radioed that the threat was not credible and cleared the plane to go to the terminal.
At Newark Liberty International Airport, passengers were removed from a United Airlines flight after it arrived from Madrid. The plane was being inspected at a spot away from the terminal.
Maryland State Police said they received an anonymous call at the McHenry barracks in the western part of the state threatening commercial aviation about 6:30 am on Monday and referred it to the FBI.