Sweden shared proof that a small foreign submarine illegally traveled in its waters last month, according to a top military officer Friday. The mysterious occurrence caused the country's largest military mobilization since the Cold War.

In October, stealth ships, helicopters and more than 200 troops were sent searching in the Baltic Sea off of the capital Stockholm after there were reports of "underwater activity" but did not find or bring up any submarine.

"The military can confirm that a small U-boat breached Sweden's territorial waters. We can exclude all alternative explanations," Sweden's armed forces head General Sverker Goransson said in a news conference.

The presence of the underwater vehicle was picked up by military sensors around the Ingaro bay, he said. However, it could not be determined, which country was behind the unauthorized passage.

In support of the claims, Goransson included a sonar image showing a bubble pattern indicating a diving submarine and tracks on the sea floor.

"We will defend Sweden's territorial integrity with all available means," Prime Minister Stefan Lofven said. He added that this intrusion was unacceptable and Sweden would be bolster detection in this area.

The Scandinavian country has already built up their defensive means, increasing military spending for up to 70 new fighter jets and new submarines. Sweden has also drawn closer to NATO recently, although it has not sought out membership of the U.S.-led alliance.

Suspicion fell initially on Russia, but the nation denies any involvement in the incident. In 1981, a Russian sub ran aground near a Swedish naval base, which was later found to be carrying nuclear torpedoes. And reports at the time of the October sub sighting claimed that Swedish intelligence had intercepted a Russian distress signal.

The hunt for the submarine elevated already rising tensions of a government already concerned about Russian aggression because of the country's role in the Ukraine crisis.