Hubertus Czernin, an Austrian journalist who was a key figure in efforts to return several multimillion-dollar paintings looted by Nazis in World War II to their rightful owner in Los Angeles, has died. He was 50.

Mr. Czernin, who had been in failing health for several years, died on June 10 in Vienna, Austria, of complications from mastocytosis, a rare blood disease, according to Randol Schoenberg, a Los Angeles attorney who represented Maria Altmann in the art restitution case.

The works in question were pieces by Gustav Klimt that were seized by Nazi officials in Vienna in 1938.

The pieces included two famous portraits of Adele Bloch-Bauer, an art patron who helped finance Klimt's work, as well as three landscapes. One portrait was sold last week for $135 million, the highest price ever paid for a painting.

Mr. Czernin was the first journalist to gain access to the archives at the Austrian Gallery, the country's national museum, and examine the paper trail surrounding pieces of art with questionable ownership.

His reporting led to a series of articles about stolen art works in Der Standard in 1998, when Mr. Czernin first wrote about the case of Adele Bloch-Bauer and her paintings.

The Art Restitution Law opened museum archives in Austria and facilitated claims from families seeking restitution of art works that were either seized by the Nazis during the war or expropriated by the Austrian government after the war.

After the legislation was enacted, Ms. Altmann received a call from a friend in Austria telling her of the development.

Over the years, Ms. Altmann had been told that her aunt, Adele Bloch-Bauer, had given the paintings to the Austrian gallery in 1925.

But Mr. Czernin's research showed that claim to be false and paved the way for Ms. Altmann's legal efforts to gain custody of the works.

In 2004, the U.S. Supreme Court ruled that Ms. Altmann could sue Austria in U.S. courts for restitution of the paintings.

First Published: June 19, 2006, 4:00 a.m.