London’s Gatwick Airport was closed Thursday during one of the busiest times of year after drones were spotted over the runway.

According to The Associated Press, the prospect of a collision between an “industrial”-grade drone and an airliner led authorities to stop all flights in and out of the airport. Police said that the intrusion was a deliberate attempt to disrupt operations at the airport during a peak period, but that the disruption was not terror-related.

Police tried to find the drone operator as soon as the first unmanned aircraft was seen over Gatwick on Wednesday evening, to no avail.

Defense Secretary Gavin Williamson says the military will be deployed to help police find the operator and stop the disruptions.

The length of the disruptions outstrips the usual battery lifespan for commercial drones, according to CNN, so the operator must have come prepared with multiple battery packs.

It is illegal to fly drones within 1 kilometer (0.6 miles) of a UK airfield.

Hundreds of travelers were stranded at the airport overnight at Gatwick – Britain’s second-busiest airport by passenger numbers; about 110,000 people had been scheduled to travel though the airport on Thursday. The runway was shut down Wednesday evening after two drones were spotted. It reopened for around 45 minutes at 3 a.m. Thursday, but was closed again after further sightings.