The military did not manage to determine the origin of the suspicious underwater sounds. According to the Maritime Guard, some of them resulted from "normal activities."
However, the military said they cannot rule out the presence of a submarine or other kinds of underwater activity.
The preliminary investigation was closed, and the case will not be taken to the prosecutor office.
The submarine hunt operation involved army and border guard vessels and helicopters.
In October 2014, Finland's neighbor Sweden accused Russia of operating a submarine in its territorial waters. An investigation by Sweden's military found that the object suspected of being a Russian submarine was a Swedish vessel.
Since then, the Swedish government decided to spend nearly $112 million on its defense, due to fears that Russia might increase its military presence in the Baltic Sea.
