U.S. Defense officials said they detected Russian submarine 275 miles from the southern U.S. East Coast. Nuclear-powered Russian attack submarine was tracked last month but a report was published only this week.

Pentagon said the submarine did not reach U.S. territorial waters. American warships were following the sub and then it allegedly changed the route heading to Europe, Russia’s NTV company reported.

However, U.S. defense officials do not know exactly the reason for submarine to come close to the U.S. coast.