An unidentified drone sighted disrupted flights at Hanover Airport in northern Germany early Tuesday morning. Based on what the airport spokesperson said, the runways were closed between 4:00 and 5:20 a.m.

Six passenger flights faced delays of 15 to 50 minutes; including flights to Turkey, Egypt, Greece, and Spain.

Hanover’s criminal police have taken charge of the investigation. Germany’s federal police declined to comment further due to the ongoing review. Spiegel, however, citing an internal police report, wrote that “it is not possible to rule out coordinated activity.”

The incident comes one week after a drone carrying explosive materials was discovered at Leipzig Airport. German officials attributed the failed attack to Russian intelligence services.