The plane, an Airbus 340, landed on Tuesday at Stockholm’s Arlanda airport.
As maintenance crews were examining the plane, they discovered a hole six millimetres in diameter on one of the plane’s wings.
“What I can say now is that there are suspicions of there being a bullet hole,” said SAS spokesperson Anders Lindström to the Aftonbladet newspaper.
"But it could be from natural causes. Unfortunately, I can’t say any more.”
Local police have been called in to examine the hole in greater detail.
“The hole is in an aileron on the wing. We don’t yet know what caused it. Our technicians are on the way to investigate,” said Yvonne Lindholm of the Arlanda police.
Lindström said the airline takes the incident “extremely seriously” and that the plane will remain out of service until the investigation is completed and the plane has been deemed safe to fly.
He tells the Svenksa Dagbladet newspaper that plane's passengers were never in danger and that the pilots didn't notice any problems with the plane.