A new wave of bomb threats targeted airlines across Europe on Thursday, forcing flights into unplanned landings and prompting fighter jet escorts, though none of the warnings has proven genuine.
No group has claimed responsibility for the alerts, and authorities have not attributed blame.
Bomb threats are not new to airlines, but security experts have noted that the timing and apparent coordination of these latest incidents are unusual.
Here is an explanation of the risks confronting airlines and passengers.
How are bomb threats made?
Bomb threats against planes are usually communicated to airports, airlines or aviation authorities using anonymous emails or phone calls, claiming that an explosive device is on board the flight.
The authorities do not always release information regarding how threats are made because it is understood that doing so could compromise investigations and encourage copycats.
A representative for Prague airport on Thursday told CNN that they had received a threat via an anonymous email addressed to the airport, and similar emails went to other European airports, Czech police confirmed.
Head of aviation security group Dyami Eric Schouten told Reuters an airline's security department first runs the report through a checklist.
Subsequently, the security department alerts the authorities, who may deploy fighter jets or divert the plane to a nearby airport for a full search.
What are the risks?
Most bomb threats turn out to be false alarms, although all are treated as live threats until thoroughly investigated.
In the latest wave of threats, no explosive device has been found, but each airline must decide whether a threat is credible and act accordingly.
A British government spokesperson described the incidents from this week as "malicious hoax threats" and said it has "one of the most robust aviation security systems in the world".
Who might be behind the hoax threats?
That is not yet clear, but it is the question on everyone’s lips.
Rhys Jones of Osprey Flight Solutions told Reuters that bomb threats could stem from a large variety of motives, including political grievances, activist campaigns, criminal activity, online groups or state-linked hybrid operations.
No evidence has currently been released linking the threats from this week to any specific state or organisation.
How do threats impact airlines and passengers?
Even false alarms and hoax threats can disrupt airline activities, with aeroplane diversions and intensive security inspections causing severe delays for their passengers.
Carriers usually bear extra costs too, including fuel, passenger rebooking, crew disruption and aircraft diversions.
A full security sweep means unloading passengers and baggage, searching the aircraft and reloading, Mr Schouten said.
That can push crews past duty limits and lead to missed connections and cancellations, he added.
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 no ask today of our governments to change the way they manage these types of situations."
Read MoreTwo UK airports received threats as bomb alerts impact flights across Europe
Flight bomb threats latest: Heathrow and Birmingham airports ‘targeted by threats’
British man being treated for Ebola in Congo is being evacuated to the UK
The Latest: Flights diverted in Europe over bomb threats
The gifts Bob Dylan and Pope Leo exchanged during meeting at Vatican
Prominent German politician found dead in Berlin hotel room hours before speech
