HELSINKI — Finland on Thursday closed an inquiry into a possible submarine incursion in April, saying it was impossible to determine whether a foreign power had violated its waters.

The Finnish navy fired two series of warning shots off the coast of Helsinki in the early hours of April 28 after detecting "underwater sounds that were noncompliant with the surface situation in Finnish territorial waters," the coast guard said in a statement.

However, the coast guard's inquiry failed to find conclusive evidence of a submarine.

"The source of the sounds relating to the initial events could not be established ... A submarine or underwater activity cannot, however, completely be ruled out," it said.

The incident came amid an upsurge in Russian military activity in the Baltic which has prompted non-aligned Finland to announce closer military cooperation with its Nordic neighbors — in particular Sweden but also with NATO members Norway and Denmark.

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