Accused Nazi death camp guard John Demjanjuk arrived in Germany from the United States on Tuesday to face charges he helped kill 29,000 Jews in 1943, raising the prospect of Germany's last major Nazi trial.
The retired auto worker had fought his deportation from the US for months but in the end, American courts rejected his appeals.
However, the 89-year-old, who tops the Simon Wiesenthal Centre's list of the top 10 most-wanted war criminal suspects, may yet be deemed unfit to stand trial.
Mr Demjanjuk, born in Ukraine, has denied any role in the Holocaust. He has said he was drafted into the Russian army in 1941, became a German prisoner of war and served at German prison camps until 1944.
A German judge in Munich issued an arrest warrant in March to put Mr Demjanjuk on trial for assisting in the deaths of 29,000 Jews at the Sobibor extermination camp during the Second World War.
Mr Demjanjuk is expected to be taken straight from the airport to a jail near Munich. Lawyer Guenther Maull has said he will advise his client to say nothing.
Mr Demjanjuk was stripped of his US citizenship after he was accused in the 1970s of being "Ivan the Terrible", a notoriously sadistic guard at the Treblinka death camp.
He was extradited to Israel in 1986, and sentenced to death in 1988 after Holocaust survivors identified him as the Treblinka guard. But Israel's Supreme Court overturned his conviction when new evidence showed another man was probably "Ivan".
Mr Demjanjuk regained his citizenship in 1998, but the US Justice Department refiled its case against him in 1999, arguing that he worked for the Nazis as a guard at three other death camps and hid the facts. His US citizenship was stripped again in 2002.