Finnish military officials say that an "underwater object" the navy chased last month in territorial waters and dropped several depth charges could have been a foreign submarine.

A navy investigation released Thursday says that technical analysis did not provide sufficient proof of the presence of a submarine during the April 27-28 search off the coast of Helsinki with inconclusive sound data, but that the existence of "of a submarine, or underwater activity" could not be ruled out.

The incident followed a lengthy hunt for a foreign submarine in the waters of neighboring Sweden in October during a period of growing military activity in the Baltic Sea region, with several reports of airspace violations by Russian military aircraft and an increased presence of Russian military vessels.