Planes were checked in Poland, Spain, Bulgaria and Italy. Two flights heading to Warsaw were among those affected, including one from London Heathrow.
In Rome, sources at Fiumicino airport told the BBC that police bomb disposal experts carried out standard security protocols to ensure the safety of passengers and crew.
The flights into Sofia were not diverted. No explosives were found when the planes were checked, according to Peev.
Bulgaria’s border police said in a statement sent to the BBC that at about 10:35 local time (08:35 GMT) it had received an email reporting “the presence of an improvised explosive device on a flight returning from Bergamo”. The same message also reported a “similar threat against aircraft arriving from Dublin”.
A passenger identified as Sarah, speaking to BBC Radio 5 Live, said her aircraft “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 said.
He said operators also weigh facilities at a diversion airport and how the aircraft can continue if the alert proves false. There are “a lot of processes” to “validate the credibility of the threat”, he said, adding that the pattern “would suggest to me that it’s been a very deliberate act by a typical actor rather than a load of coincidental, malicious calls”.
Chan said crews will “quietly search” toilets, galleys and storage areas and will not touch a suspicious item unless it must be moved to an area designated as the least risky bomb location. Once on the ground, he said, the aircraft is generally “taken to an isolated stand, passengers are disembarked, and police and bomb disposal teams search it”.
“We weren’t told anything during the flight,” she added. “We were only later told that it was an actual threat, a bomb threat to the plane itself.” She said the plane had not been on the ground a minute before it was “surrounded by fire engines and police and a bomb squad”.
BBC Verify consulted aviation safety experts on procedures when a bomb threat is issued. Mason said a flight may be asked to divert if the credibility of a threat “is of a certain level”, and that in some cases onboard crew may not be told what ground operators were dealing with.
In a separate development, a bomb alert prompted the evacuation of 1,537 passengers and crew from an Israel-bound cruise ship in Greece, media reports from both countries said.