London’s Heathrow Airport halted departure flights after a possible drone sighting, the airport announced on Twitter.

“We are responding to a drone sighting at Heathrow and are working closely with the Met Police to prevent any threat to operational safety,” read the tweet. “As a precautionary measure, we have stopped departures while we investigate. We apologise to passengers for any inconvenience this may cause.”

The news comes several weeks after a drone sighting caused several days of flight disruptions at London’s Gatwick Airport. Two individuals were arrested following an investigation, although they were later released without charge. It also is the first test of Heathrow’s newly acquired Israeli-built drone defense system, which was installed following the Gatwick incident.

Updated January 8th, 1:54pm ET: Departure flights have resumed at Heathrow, while airport officials continue to work with local law enforcement, the airport said less than an hour after first announcing the shutdown.