“As a precautionary measure, we have stopped departures while we investigate,” an airport spokesperson said in a statement. “We apologize to passengers for any inconvenience this may cause.”

All flights were resumed just over an hour after the suspension, The Telegraph reports, with Heathrow officials continuing to work with Air Traffic Control and the Metropolitan Police to monitor the situation.

Heathrow is Europe’s busiest airport, with nearly 78 million travelers passing through in 2017.

Drones can cause major damage to an airplane, prompting airports to take any nearby sightings seriously. A similar incident closed Gatwick Airport, the UK’s second busiest airport, in late December for more than a day.

Located roughly 35 miles southeast of Heathrow, Gatwick shutdown on December 19 when two drones were spotted flying into the runway operations area. The airport remained closed for much of the following two days as police and armed forces attempted to locate the drone operator.

Hundreds of flights were affected by what police called a “deliberate act” of disruption, BBC reports.

The drone interruption at Heathrow came just after the airport announced a controversial plan to add new arrival flight paths, expanding its capability to welcome 25,000 more flights a year.