Sweden says it can confirm a submarine violated its territorial waters near Stockholm in October.
“There is no doubt, we have excluded all other explanations. Swedish territory has been seriously and unacceptably violated by a foreign power,” said Supreme Commander Sverker Göranson in a statement.
The statement did not disclose the nationality of the submarine, but it is widely believed to have been Russian, a theory that was fueled by local news reports that Swedish military intelligence intercepted radio signals off the coast of Sweden to Kaliningrad, home to much of Russia's Baltic fleet, and that they were transmitted on a special emergency frequency.
Russia denied one of its vessels was involved.
The hunt for the submarine, which was first spotted on Oct. 17, was the biggest mobilization of Swedish military forces since the Cold War, and included 200 men, stealth ships, minesweepers and helicopters. The search lasted a week.
Swedish military officials called the evidence they collected highly credible, and included a photo of an object taken by a “member of the public” that showed an object moving at a speed of 1 knot. The photo, they said, showed a spray of water characteristic of submarine movement.
In this amateur photo provided by Sweden's armed forces and distributed by the TT News Agency on Sunday, Oct. 19, 2014, a partially submerged object is visible in the water at center, in the Stockholm archipelago, Sweden.
Another civilian observed “an underwater body with distinctive features,” the statement said.
Swedish armed forces also collected data that they said generated a pattern confirming “a foreign power has violated Swedish territorial integrity.”
“The gravity of this is obvious,” said Göranson.
Sweden’s prime minister, Stefan Lofven, was quoted by The Associated Press calling the incursion “totally unacceptable.”
Richard Kauzlarich, a senior fellow at the Brookings Institute, said the revelation was not surprising given the pattern of “aggressive” Russian military actions, but he did say those actions could end up costing Moscow.
“The risk to Moscow however is twofold: first, the unannounced incursions could result in an accident involving civilian aircraft or shipping,” he said. “Second, rather than dividing the NATO allies and non-NATO states like Sweden, it actually brings them closer together.”
The disclosure by Sweden, which is not a NATO member, comes amid rising tensions in the Baltic region over Russian military incursions.
Earlier this week, the European Leadership Network (ELN), a London-based think tank, issued a report saying Russia’s provocative actions in the Baltic region were at Cold War levels.
NATO said they have tallied over 100 separate incidents this year in which NATO jets have been scrambled to intercept Russian planes, more than three times as many as last year.