Running silently at periscope depth, 66 feet under the sea, the U.S. nuclear-powered attack submarine was probably on a spy mission far north of the Arctic Circle near Murmansk, fewer than 20 miles from strategic navalbases of the former Soviet Union.
Farther below the icy Barents Sea's surface, a nuclear-powered submarine of the former Soviet Union was slipping through deeper water near home port.
Then it happened. The Russian sub began to surface and crashed into the U.S. sub.
In peace time, the attack submarine watches other submarines. In times of war, its mission is to destroy them. But never are they supposed to collide.
Beyond admitting that it happened, the U.S. Navy is offering no details of the collision, not even a clue about the obvious question: How can two sophisticated submarines with high-tech electronics equipment ever run into each other? As it turns out, in crowded sea lanes, this was not a first.
No injuries were reported aboard either submarine. And there were no measurable radioactive releases to the Barents Sea outside the Russian port of Murmansk, 1,000 miles north of Moscow.
The crew of the American sub, the Baton Rouge, did not detect any damage. But Russian news reports said metal parts were found on the deck of the Russian sub. As a precaution, the U.S. sub is heading to its home port of Norfolk, Va. It is due there Tuesday.
After the collision, the American submarine watched the Russian sub sail toward port but did not contact it by radio.
Both subs are capable of carrying nuclear weapons, but neither side would say whether any were aboard at the time. Indications are that the American sub, at least, was not nuclear-armed. The U.S. sub is of the Los Angeles class, a very fast attack submarine.
Less than three weeks ago, the United States and Russia declared they are no longer potential enemies. So why were these submarines so close?
The United States might be seeking to improve its knowledge of operations of the former Soviet navy, whose chain of command and possible division is still being negotiated among the former Soviet republics making up the new Commonwealth of Independent States.
Another possibility is that the American submarine, which the Navy said was sitting at "periscope depth," might have been using secret interception equipment to monitor communications at nearby military bases.
"If you were wanting to watch activities and post a sentry, that's where you'd stand, at the busiest intersection," one U.S. official said. "It's a common-sense kind of thing to do."
Military analysts say satellite intelligence-gathering is difficult along the Kola Peninsula because of fog and clouds. The Northern Fleet is headquartered on the peninsula, and several major submarine bases also are in the area, according to military analysts.
Moscow complained that the United States submarine was in restricted waters. The Pentagon dismissed those accusations.
The question could hinge on definitions.
Soviet naval chart makers traditionally have measured their 12-mile belt of territorial waters as extending outward from the mouths of smaller bays and harbors, rather than from the irregularities of the coastline.
Interfax, a Russian news agency, said that in this instance, Russia defines its territorial waters as extending 12 miles out from the mouth of Kola Bay, the icy harbor outside the Murmansk naval base.
U.S. and Soviet submarines have long engaged in cat-and-mouse games in the busy Arctic waters where the accident occurred, and collisions are not unheard of.
The most devastating accident reported so far involved a collision in 1970 between a U.S. attack sub and a Soviet missile-firing sub it was shadowing in the Northern Pacific. In that incident, the Soviet sub sank, according to an investigative account in the Chicago Tribune last year.
The newest collision occurred Feb. 11. First word of the news came out Thursday, when the Russian news agency Itar-Tass reported a sub's "collision with an unknown underwater object."
Through military channels, Defense Secretary Dick Cheney learned of the incident the next day. In Moscow on other business Monday, Secretary of State James Baker broke the news to Russian President Boris Yeltsin: The hidden obstacle was an American sub.
The incident is unlikely to disturb relations between the United States and Russia, which announced plans on Tuesday for military cooperation to warn of ballistic missile attacks.
But despite improved relations, the affair shows that there is still an undercurrent of tension between the United States and the military of the former Soviet Union.
On Tuesday, Navy officials sought to pre-empt criticism of the submarine patrols, asserting that as long as Russia continues to deploy submarines, the Navy has an obligation to keep track of them.
The surface fleet of the former Soviet Union virtually has ceased operations for lack of fuel. But the story might be different for its submarines.
Adm. Frank Kelso, the U.S. Navy's top uniformed officer as chief of naval operations, refused to discuss the incident. But he stressed that the U.S. fleet would continue to operate as usual in international waters.
In times of high military tension, the waters off the Kola Peninsula would be a sensitive area. American submarine warfare doctrine calls for using mines and torpedoes to sink any Russian vessels that try to leave port during a war.
Private U.S. analysts said it seemed likely the Baton Rouge was on a reconnaissance mission, and some questioned the need for continued U.S. surveillance of Russian waters at a time of dramatically improved political relations.
"The question is, why are we doing this kind of operation now? The Cold War is supposed to be over," said James T. Bush, a retired Navy captain and submariner. "They really haven't adjusted to the fact that the Cold War is over."
_ Information from the New York Times, the Washington Post, Cox News Service and the Associated Press was used in this report.