LONDON — The mystery vessel was found off the Swedish coast last week lying on the sea floor, its hatches closed. Cyrillic letters on the hull suggested it was Russian.

In Sweden, where memories of the Cold War run deep, speculation swirled about whether the unidentified craft was a Russian submarine that had sunk while on a secret mission. Its age was not immediately obvious, and the Swedish authorities were unsure how long it had lain there, submerged and undetected.

This week, the Swedish authorities said they had solved the mystery: The vessel was an imperial submarine — nicknamed a Catfish — that had probably sunk after colliding with a Swedish vessel in 1916, during World War I.

The Swedish news media reported that the submarine had been part of the Imperial Russian Navy and that it had sunk with its crew of 18 in May 1916.