Waves of bomb threats have so far proved to be hoaxes, leaving officials searching for answers about who is behind them—and why.

Dozens of flights across Europe have been targeted by bomb threats this week, causing diversions and emergency landings.

A fresh wave of bomb threats disrupted air traffic across Europe on Friday but the affected aircraft landed safely and searches revealed no explosive devices. The latest alerts affected at least 18 flights to or from Bulgaria, Cyprus, Czechia, France, Hungary, Italy, Portugal, Romania and the UK. Danish police investigated a threat against Copenhagen airport but the airport’s management said it was operating normally. The warnings followed separate threats on Tuesday and Thursday that forced at least 35 flights to be diverted or searched, raising questions over an apparently coordinated effort to cause chaos for travelers . A number of European media outlets – including news organizations in Bulgaria, Ireland and the UK – have reported that a group claiming responsibility for this week’s threats sent an email to several outlets warning of further attacks on transport across Europe. “This is only the beginning,” it reportedly said. The latest threats affected at least nine flights to and from Prague, four flights…