Drone sightings prompted Sweden to temporarily halt air traffic at Stockholm’s Arlanda airport overnight, police said on Monday.

Police have launched an investigation into possible airport sabotage, they said in a statement.

Traffic at Arlanda, Sweden’s biggest airport, was halted around 2300 GMT and resumed at 0130 GMT, they added.

Daily Aftonbladet quoted an aviation authority spokesperson as saying four drones of varying sizes were sighted from the air traffic control tower.

“A decision was then made to halt take-off and landing,” the spokesperson said.

A spokesperson for airport operator Swedavia confirmed the temporary halt but declined to comment further. The aviation authority was not immediately available for comment.