"We suspended the exercises because there were indications of unknown underwater activity," navy spokesman Jesper Tengroth told the Swedish newspaper Svenska Dagbladet, without disclosing the exact nature of the indications for security reasons.
Tengroth subsequently explained that the navy term "unknown underwater activity" had a broad definition and did not apply exclusively to submarines.
Later, the Swedish military confirmed that "a mini submarine" had supposedly violated its territorial waters, but was never able to establish the vessel's nationality and never came with more substantial proof than grainy photos taken on mobile phones.
In July 2015, a mini submarine was found by divers off the coast of Sweden amid anxiety over Russia's alleged surveillance operations in the Baltic. However, this "sensation" turned out to be a Tsarist-era Som-class submarine which sank in 1916 during WWI. Since then, Sweden has never tired of accusing Russia of sending submarines into Swedish territorial waters, obviously using it as a pretext for inflating its military budget.
Of late, non-aligned Sweden has also markedly stepped up its cooperation and joint exercises with NATO. Presently, Sweden takes part in two US-led maneuvers in the Baltic (BALTOPS 2016 and Anakonda 2016). In this year's edition of BALTOPS, Sweden will provide NATO forces with a springboard in Stockholm's archipelago, whereas US bombers B-52 will fly across Sweden's airspace.