SINGAPORE (AP) — Drones buzzing around Singapore’s Changi Airport have caused the delay or diversion of 63 flights in the past week, triggering an official investigation and raising questions about the motives of the offenders.

Regulators said Tuesday that 18 flights at the airport were delayed and seven were diverted the night before “due to bad weather and unauthorized drone activities.”

It had earlier confirmed sightings of drones flying near the airport last Tuesday and Wednesday. That caused the intermittent closure of a runway, delaying 37 flights and diverting one arriving plane.

Investigations are ongoing.

Drones cannot be flown within a several kilometer (mile) radius of airports or military bases in Singapore.

The offence carries a maximum penalty of a year’s jail term and a fine of 20,000 Singapore dollars.