Transcript for The Hunt For a Russian Nuclear Submarine
And to a puzzle that seems like it's right out of the cold war. The hunt is on tonight for a possible Russian submarine. Sweden is convinced it is in their waters. Look at this. It's an image. There is a Russian submarine lurking below? ABC's chief foreign correspondent terry Moran tonight on the search and what the Russians are now saying. Reporter: Somewhere underneath these waves, the Swedish government believes an intruder lurks. Perhaps a Russian submarine. It's a cold war game of cat and mouse, just like the one in that movie "Hunt for red okts." Do we have any surface contact? No contact, sir. Reporter: For four days, Swedish Navy vessels equiped with powerful sonar and anti- submarine weapons, have scanned the countless bays and inlets here. But so far, just a few fleeting, tantalizing clues. This photo, taken by a civilian outside Stockholm -- is it a partially submerged sub? An encrypted communications, the Swedes claim they intercepted to a Russian base on an emergency frequency. If there is a submarine in trouble that could be a nuclear powered submarine, that's very close to Swedish waters, then that could be a very serious problem. Reporter: Moscow is denying everything. But the Swedes aren't buying it. Like so many of Russia's neighbors, they don't trust Vladimir Putin. And so, this October hunt will continue.
This transcript has been automatically generated and may not be 100% accurate.