Fifty-five years ago this month Soviet agents operating in Hungary arrested a young Swedish diplomat. Raoul Wallenberg already had become a legend for saving tens of thousands of Hungarian Jews from Nazi death camps in the final months of the war. His disappearance remains one of the greatest unsolved mysteries of the 20th century.

Wallenberg was only 32 years old and without any diplomatic training when dispatched to Nazi-occupied Hungary in July 1944, to take on the mission of saving as many of Budapest's remaining 200,000 Jews as possible. During his six months in Hungary, Wallenberg pursued that mission passionately and with extraordinary ingenuity. He created the Schutzpass, a Swedish protective document, which he distributed with the help of a small group of assistants to thousands of Jews whom he found on deportation trains or on death marches. And, remarkably, he faced down the notorious Adolf Eichmann and the Nazi occupying forces in Hungary to help prevent a pogrom in Budapest's central ghetto.

Only one week before his arrest, Wallenberg told a colleague: "For me there is no choice. I've taken on this assignment, and I'd never be able to go back to Stockholm without knowing inside myself I'd done all a man could do to save as many Jews as possible."

It is estimated that Wallenberg's heroic efforts saved the lives of 100,000 Jews. He was an angel of mercy--and that is why his disappearance continues to haunt us.

Why was Wallenberg arrested on Jan. 17, 1945, and what happened to him after he entered the infamous Soviet gulag?

These answers are hidden in Russia--in Soviet-era archives that remain shut and in the minds of aging witnesses who have yet to reveal what they know. Scholars accessing Soviet-era archives in recent years have found them remarkably well maintained. The Wallenberg files most likely were given the same meticulous care. During the Soviet era, inquiries about Wallenberg were met with alternating and contradictory responses. The Kremlin first acknowledged that he was in Soviet hands, then denied any knowledge of him, then claimed he had died of a heart attack and finally asserted that his death could not be explained. Since the collapse of the Soviet Union in 1991, Moscow has been silent. Rumors abound, but to this day we do not know with certainty whether Wallenberg is alive or dead.

What makes Wallenberg's arrest and disappearance inexplicable is the fact that the Soviet Union was an American ally during the war and suffered enormous losses in its effort to help destroy the Nazi regime. This makes the need to know all the greater.

The United States has a special interest in pressing Moscow to solve this puzzle. While working in the Swedish Embassy in Budapest, Wallenberg was employed by the U.S. War Refugee Board, belatedly established in 1944 by the Roosevelt administration to try to save European Jews.

In 1981, through an initiative of Rep. Tom Lantos (D-Calif.), whose wife, Annette, was one of the thousands rescued by Wallenberg, Congress granted Wallenberg honorary U.S. citizenship. It was only the second time in history that Congress had taken such action. The U.S. Postal Service issued in 1997 a postage stamp in his honor, and his bust stands in the Capitol Rotunda.

But Washington has yet to adequately press Moscow at the highest levels to open the Wallenberg file. As William Korey, a scholar on Soviet affairs, commented recently: "It is now 55 years since a U.S. secretary of state expressed, however indirectly, any concern about Wallenberg's disappearance."

Far too much time has passed. Solving the Wallenberg mystery must now become a priority in our bilateral relations with Russia. Acting Russian President Vladimir Putin can demonstrate his goodwill by opening fully the Soviet-era files in which the answers--and the truth about Wallenberg's fate--can be found.

The writer is executive director of the American Jewish Committee.