Allan A. Ryan, Dogged Pursuer of Nazi Collaborators, Dies at 77

“He really was the groundbreaker,” Ms. Holtzman said in a phone interview. “He set the standard.”

Most of Mr. Ryan’s targets were low-level guards and functionaries, but he also went after relatively high-level targets. Andrija Artukovic had been the interior minister in Nazi-occupied Croatia, where he oversaw the deaths of hundreds of thousands of Jews and Serbs. He was deported to what was then Yugoslavia in 1986, and died in prison.

Valerian Trifa was a Romanian fascist who had fomented anti-Semitic pogroms, but after immigrating became the archbishop of the Romanian Orthodox Church in the United States and Canada. After Mr. Ryan’s office filed charges, Mr. Trifa renounced his citizenship and, to avoid deportation, moved to Portugal, where he died in 1987.

In 1983 William French Smith, the attorney general, assigned Mr. Ryan to investigate charges that the U.S. Army had helped a former German SS officer, Klaus Barbie, escape from France, where he was wanted for torturing prisoners, in exchange for providing intelligence. Mr. Barbie had settled in Bolivia but was later extradited to France, where he was sentenced to life in prison.

Mr. Ryan’s report was searing. While he conceded that there might have been a national-security reason for employing Mr. Barbie, it had been morally unconscionable to help him escape French justice. He recommended that the U.S. government apologize to France, which, in a rare instance of diplomatic humility, it did.

His most controversial case involved a retired Cleveland autoworker named John Demjanjuk. Eyewitness testimony indicated that he had been the so-called Ivan the Terrible, an especially barbarous guard at the Treblinka camp. The Office of Special Investigations won his deportation to Israel, where he was sentenced to death.

But after the fall of the Soviet Union, evidence emerged that Mr. Demjanjuk had not been Ivan the Terrible; Israel released him in 1991 and he returned to Cleveland. Then, during a subsequent inquiry, former lawyers at the Office of Special Investigations testified that Mr. Ryan and others had withheld evidence calling into question Mr. Demjanjuk’s identity.

Mr. Ryan, who had long since left the federal government, admitted that the evidence had not been shared with Mr. Demjanjuk’s defense team, but that it had been an oversight. A U.S. District Court judge overseeing the inquiry agreed, though another court denounced the office’s “win at any cost” attitude.