Air France Flight 65 from Los Angeles International Airport to Charles de Gaulle Airport in Paris was diverted to Salt Lake City International Airport, Air France said in a statement. At about the same time a second flight, Air France 55, took off from Dulles International Airport outside Washington and was diverted to Halifax on Canada's East Coast, officials said.
Passengers got off both planes safely and were taken to terminals.
American authorities investigated and found no credible threat, according to an FBI statement released late Tuesday night.
The Royal Canadian Mounted Police said investigators found no evidence of an explosive device after they searched the plane and luggage.
Passengers in the Utah airport were boarding their plane again around 11:30 p.m., Salt Lake airport spokeswoman Bianca Shreeve said.
Keith Rosso of Santa Monica, California, a passenger on the flight from Los Angeles with his fiancee, said "everything was smooth, everything was great, everything was going swell" for the first two hours of the flight, then things changed.
"The flight attendants quickly came by and cleared plates, then there was an announcement that we were making an emergency landing and that the flight attendants were trained exactly for situations like this," Rosso told The Associated Press by phone from the airport in Salt Lake City.
He said he looked at the flight monitor at his seat and saw that "we had made a pretty sharp right turn - we had been almost near Canada - toward Salt Lake City."
Rosso said an FBI agent interviewed the passengers after the landing.
In Halifax, the Royal Canadian Mounted Police led the investigation.
RCMP Constable Mark Skinner said there were 262 people onboard that plane, which also received an anonymous threat. No further details on that threat were released.
"We received a complaint of a bomb threat and we responded to it," Skinner said.
Halifax Stanfield International Airport spokesman Peter Spurway said police cleared the plane. He said passengers will go through Canadian customs, pick up their baggage and be put up at hotels overnight.
"Air France will make a decision as to when it will depart," Spurway said.
The threats came after last week's attacks in Paris that killed 129 people and heightened security concerns around the world.
Associated Press Writers Stephanie Siek in New York, Rob Gillies in Toronto and Andrew Dalton and Michelle A. Monroe in Phoenix contributed to this report.