Seven passenger planes across Europe, two of which departed from the UK, have been inspected following a series of suspected bomb threats.
Two Ryanair flights, one originating from Dublin and the other from Lisbon, were searched after landing in Rome following anonymous tips but no explosives were found, sources at the city's Fiumicino airport told the BBC.
The Dublin flight was escorted by the Italian Air Force until landing. Authorities have also searched planes in Poland, Spain and Bulgaria.
While officially there is no link between them, authorities from various countries will be co-operating to assess whether this was a co-ordinated attack, designed to disrupt airspace.
The UK government described today's incidents as "malicious hoax threats".
In a statement it said it had "one of the most robust aviation security systems in the world, with well-tested processes for dealing with any potential threats, as well as malicious hoax threats, like those we've seen today".
Aviation security expert Paul Mason said bomb threats on passenger planes "don't happen that frequently", which made the latest incidents "unusual".
"It sounds like they've been made in a credible manner across a broad range of airlines," Mason told BBC Radio 5 Live. "That would suggest to me that it's been a very deliberate act by a typical actor rather than a load of coincidental, malicious calls."
He said "there are a lot of processes" that the onboard crew and people on the ground must go through "to validate the credibility of the threat".
A flight may be asked to divert if the credibility of a threat "is of a certain level", Mason added, and in some cases the onboard crew may not even be advised on what the ground operators were dealing with.
In Rome, sources at Fiumicino airport told the BBC police bomb disposal experts carried out standard security protocols to ensure the safety of all passengers and crew.
Inspections of the passengers, luggage, and the aircraft yielded negative results, and the alert has since been cleared.
One plane which landed in Murcia, southern Spain - a Ryanair flight coming from Manchester - was also found not to have any explosives on board, Spanish authorities said.
One passenger later said she was unaware of the bomb threat until after the plane landed.
"We weren't told anything during the flight," Sarah told BBC Radio 5 Live. "The plane stopped quite suddenly and then we were kept on the runway for around 30 minutes."
"Then the fire engines turned up, the bomb squad turned up and we were asked to evacuate," she continues.
Sarah said the plane had not been on the ground a minute before it was "surrounded by fire engines and police and a bomb squad". All the passengers were then escorted by police, leaving all their belongings.
"We were only later told that it was an actual threat, a bomb threat to the plane itself," she added.
Two flights heading to Warsaw were also affected, including one from London Heathrow.
The Heathrow flight was originally meant to land at the Polish capital's main airport, Chopin, but was diverted instead to the smaller Modlin airport, briefly forcing its closure.
A Scandinavian Airlines flight from Copenhagen to Warsaw-Chopin landed as planned.
Later on Tuesday, Bulgarian Transport Minister Georgi Peev said there were bomb threats against two planes travelling to the country's capital Sofia.
He told the state-owned Bulgarian News Agency (BTA) the flights were from the northern Italian city of Bergamo and Ireland's capital Dublin.
The flights into Sofia were not diverted and no explosives were found when the planes were checked, Peev said.
Bulgaria's border police said in a statement sent to the BBC that at around 10:35 (08:35 GMT) it had received an email about "the presence of an improvised explosive device on a flight returning from Bergamo".
"Standard procedures for such incidents were immediately activated, and an operational headquarters comprising representatives of various institutions was convened," the statement said.
The border police added that the same email also reported a "similar threat against aircraft arriving from Dublin".
