Drone sightings shut down London’s Gatwick Airport on Wednesday night and the second-busiest U.K. airport remained closed on Thursday as law enforcement investigated what it called a “deliberate act.”

The incident stranded thousands of passengers during the busy Christmas travel period, as flights were grounded, while others were diverted to other airports. Gatwick serves more than 100,000 passengers a day, making it the second-busiest in the country after London Heathrow.

The drone flights near Gatwick’s airfield appeared to be “deliberate” but not an act of terror, according to Sussex Police.

https://twitter.com/Gatwick_Airport/status/1075756803747647489

The incident highlights concerns among air safety regulators about preventing drones from flying close to aircraft and airfields, as the devices become more popular and easily obtainable.

# GatwickDrones| We are appealing for information to help us identify the operators of the # Gatwick# drones. If you know who’s responsible or have any information please call 999 and quote ref 1350-19/12. Please R

Airlines offered passengers free flight changes due to the disruption. Budget carrier easyJet said it would reimburse travelers’ “reasonable expenses” who arrange for alternative transportation.