Why Air France flight from San Francisco was diverted to Montreal

An Air France flight from San Francisco to Paris was diverted to Montreal after an unspecified anonymous threat, and the passengers landed and disembarked safely before authorities declared it a false alarm.

At least 15 fire trucks and police cars met Flight 83 at the Montreal airport, and Canadian authorities verified the aircraft, passengers and baggage.

Air France tweeted Tuesday: "after a full security search, false alert confirmed by local authorities following an anonymous threat." Earlier, the airline had said the crew decided to divert "as a precautionary measure."

Air France said an investigation will be carried out into the origin of the threat, and promised to send passengers safely to their destinations. Spokeswoman Ulli Gendrot said all passengers were safely off the plane. She would not elaborate on the nature of the threat.

Airlines around the world face sporadic threats and occasionally divert planes as a result. On Nov. 17, an Air France flight with 262 people aboard was on its way from Washington, D.C., to Paris when a false threat was phoned into the airline and the plane changed course. After the plane landed in Halifax, Canadian police searched the aircraft and found no signs of explosives.

Tensions are unusually high in France, which has been in a state of emergency since Islamic extremist suicide bombings and shootings in Paris on Nov. 13 that killed 130 people, the deadliest attacks on French soil in decades.

Passenger Gilles Raymond of San Francisco said after about five hours in the air, the pilot made an announcement that they needed to land in Montreal for technical reasons. He said once the plane had landed, the pilot said authorities would have to do a thorough security check of the plane and passengers and that everyone would have to disembark.

"Everyone is pretty relaxed," Raymond said while still inside. "There was no panic, no stress. Everyone is waiting in their seats. Some people are sleeping, and some are checking their phones or eating ice cream that the flight attendants gave them."

He said passengers spent at least an hour on board the plane during verifications.

"They told us they will check every passenger and that we may spend the night here," Raymond said.

In the wake of the Paris attacks and the downing of a Metrojet airliner over Egypt, there has been heightened security at US airports, reports The Christian Science Monitor.

 Transportation Security Administration’s (TSA) reliance on airport operators to vet aviation workers – a matter that drew attention following the crash of a Russian charter plane over the Sinai peninsula late last month. Concerns that a bomb may have brought down the plane, and that attackers could have taken advantage of security gaps at the Sharm el-Shiekh airport to get an explosive device aboard, led US national security officials to scrutinize the airport security process here.

The nation’s 450 airports use contractors from the TSA, which oversees travel security in the US, to check employees’ names against terrorism databases, and review their immigration status and criminal histories.

Which is why despite the fact that US airports are seen as models for security worldwide, the screening process for workers has caused some to worry, a US official with knowledge of American aviation security told CNN last week. 

Meanwhile, Air France will resume flights to Tehran for the first time in more than seven years, as part of resuming European trade with Iran following a hard-fought deal to curb its nuclear activities.

The airline announced in a statement Tuesday that it "is supporting the resumption of commercial exchange with Iran" and will operate flights three times a week starting in April.

The statement says "Air France is illustrating its ambition to expand in a country with buoyant growth, as the European Union is Iran's fourth most important economic partner."

Air France operated flights to Tehran from 1946 until October 2008, when they were suspended amid U.N. and EU sanctions against Iran over its nuclear program.

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Olga R. Rodriguez in San Francisco contributed to this report.