Airlines across Europe have received waves of bomb threats this week, with flights making unplanned landings and others escorted by fighter jets, though none of the threats has proved genuine.

There has been no claim of responsibility and officials have not attributed blame.

Airlines are not new to bomb threats, but aviation security experts say the apparent coordination and timing of the current incidents is unusual.

Here, we look at the risks airlines and passengers face.

HOW ARE MOST BOMB THREATS MADE?

Bomb threats against planes are often communicated to airlines, airports or authorities through anonymous emails or phone calls claiming that an explosive device is on board. 

Authorities do not always disclose how threats are delivered because doing so could encourage copycats and compromise investigations.

A Prague airport representative told CNN on Thursday that they had received a threat from an anonymous email addressed to the airport, and Czech police said similar emails went to other European airports.

Eric Schouten, head of aviation security group Dyami, told Reuters that an airline's security department first runs the report through a checklist. It then alerts the authorities, who may scramble fighter jets or divert the plane to a designated airport for a full sweep.

WHAT IS THE RISK?

The vast majority of bomb threats turn out to be false alarms, though all must be treated as real until investigated.

No explosive device has been found on the planes affected this week, but every airline must decide whether a threat is credible and act accordingly.

A spokesperson for the British government called this week's incidents "malicious hoax threats" and said it has "one of the most robust aviation security systems in the world".

WHO MIGHT BE BEHIND IT?

Nobody knows.

Rhys Jones of Osprey Flight Solutions said bomb threats can stem from a wide range of motives, including political grievances, activist campaigns, criminal activity, online groups or state-linked hybrid operations.

There is currently no public evidence linking this week's incidents to any specific state or organization.

WHAT IS THE IMPACT FOR AIRLINES?

Even a false alarm can disrupt airlines' operations, with plane diversions and thorough security inspections causing delays for passengers. Carriers typically bear additional costs including fuel, passenger rebooking, crew disruption and aircraft diversions.

Schouten said a sweep means unloading passengers and baggage, searching the aircraft and reloading. That can push crews past duty limits and lead to missed connections and cancellations.

Air France-KLM Chief Executive Ben Smith said on Thursday he had never previously seen threats reported in such a coordinated fashion.

"We're quite confident that we have in place what we need to ensure that it has minimal or no impact on our operation," Smith told reporters. "So we have to ask today of our governments to change the way they manage these types of situations."