The other submarine was the Memphis, a nuclear-powered attack submarine based in Groton, Conn. It was one of two American submarines spying on the largest Russian naval exercise in years when disaster struck the Kursk the morning of Aug. 12.
By the time the Memphis reached Bergen, Norway, officials in Russia had said the Kursk probably had sunk after colliding with a foreign submarine or a World War II mine.
Publicly, the Pentagon refuses to comment on the whereabouts or the mission of the Memphis, but some officials now acknowledge its spying mission. They insist that the Memphis was not damaged and that no other submarine was involved in any collision. The Memphis' arrival in Norway was a long-scheduled liberty call, they said.
But the call allowed the submarine to unload sonar tapes and other recordings that the Americans say captured two explosions that ravaged and sank the Kursk, killing all 118 people on board.
Those tapes, being analyzed at the National Maritime Intelligence Center in Suitland, Md., contain the strongest evidence to support the leading U.S. theory of what destroyed the Kursk, the officials said.
And that theory, they said, does not include the collision that the Russians have said probably occurred.
"We have subs that hear everything that goes on," a senior officer said. "It's pretty clear to us what happened."
According to the U.S. theory, a rocket-propelled torpedo being loaded or launched as part of an exercise misfired, its engine or its fuel exploding. After 2 minutes and 15 seconds â€“ during which time the Kursk's captain either increased power from the nuclear reactor or blew ballast in an effort to surface â€“ a powerful explosion of the torpedo's warhead tore a gaping hole in the submarine's bow, killing most if not all of the crew instantly.
In Vladivostok, Russia, a former submarine officer who is a member of a governmental commission investigating the explosion said Monday that a new weapons system was being tested on the Kursk. But the former officer, Sergei Zhekov, would not elaborate during a news conference about the system, saying that it was a state secret, the Interfax news agency reported.
When the Kursk sank, the United States knew about it within hours. The Americans collected telltale recordings from submarines, a surface ship and even from shore. They detected no sounds of a collision. And they monitored the Russian fleet's emergency radio transmissions closely in the aftermath, officials said.
In addition to two submarines, the Navy had also sent a surface ship to the Barents, the Loyal. The Loyal is part of a class of surveillance ships that are operated by civilian contractors, but include as many as 15 Navy seamen and officers.
Within hours of the explosions, the two American submarines radioed messages back to fleet headquarters. "They were alive and well and had no bumps," another senior officer said.
According to the U.S. officials, neither the two submarines nor the Loyal detected any sounds that would suggest the Kursk had been involved in a collision of any sort. Even at great distances, the signals created by a collision or an explosion are easy to distinguish, the officials said.
U.S. officials and submarine experts said it was possible that some of the crew â€“ perhaps 15 men or more â€“ survived the initial explosions if they managed to quickly shut the watertight doors to their compartments.
The Russians said that they had detected tapping sounds from within the Kursk at least two days after it sank, raising hopes that a rescue might be possible.