As to whether the weapons used any of the U-234's uranium, Dr. Goldberg, like several historians and nuclear experts, said in an interview that he was unconvinced but intrigued. Even if none of the submarine's cargo went into the explosive mix, some experts hold, the U-234 episode is important to explore for what it reveals about the Japanese atom bomb program, which has long been clouded in ambiguity.

Dr. Peter D. Zimmerman, a physicist who has studied the issue and advises the Defense Department, said, "Where it becomes very significant is if it helps demonstrate that the Japanese had a sizable program and that there was close cooperation among the Axis powers."

The U-234 left the German port of Kiel on March 25, 1945, bound for Japan on a voyage around the horn of Africa. After Hitler's death a month later, the submarine surrendered to American forces in the north Atlantic and was taken to the American submarine base at Portsmouth, N.H., where reporters watched its arrival on May 19, 1945.

In secret, the Navy took careful inventory of the submarine's crew and cargo, recording the details in an exhaustive manifest that today is in a public file at the Navy's Operational Archives at its Historical Center in Washington.

The manifest says that the uranium came in 10 cases, weighed 560 kilograms and was transported from Germany as an oxide, which is a handy industrial form refined from raw uranium ore. Kathy Lloyd, an archivist at the center, said in an interview that the Navy had "no paper trail" on where the shipment went after the inventory.

Dr. Zimmerman said that amount of uranium oxide would have contained about 3.5 kilograms of the isotope U-235, which is the critical one for making bombs. That 3.5 kilograms, he added, would have been about a fifth of the total U-235 needed to make one bomb.

Among the experts who have tried to track the mysterious shipment is Robert K. Wilcox, a journalist and author of "Japan's Secret War" (Marlowe & Company), a book about Tokyo's atom bomb project. After its publication in 1985, he updated the book for a 1995 edition coincident with the 50th anniversary of the atomic bombing of Japan.