Carl Friedrich von Weizsaecker, 94, a physicist who helped research atomic weapons for the Nazis during World War II, but later dedicated his life to pacifism and philosophy, died Saturday in Berlin, his family said.

Dr. Weizsaecker's family gave no cause of death, but said he had been severely ill for a long time.

Born in Kiel on June 28, 1912, into a nationally prominent family of jurists and theologians, Dr. Weizsaecker studied physics and mathematics in Leipzig, Berlin and Goettingen, and went on to become a professor of physics. His brother Richard served as German president from 1984 until 1994.

Dr. Weizsaecker said he worked on the atomic bomb to avoid being conscripted into the Nazi army. He also insisted in postwar interviews that he was grateful the nuclear technology was never used by the Nazis.

But a secret recording of German scientists captured by the Allies caught Dr. Weizsaecker saying, after hearing of the U.S. nuclear bombing of Japan that, "If they were able to finish it by summer '45, then, with a bit of luck, we could have been ready in winter '44-45."

After the war, Dr. Weizsaecker dedicated his energies to philosophy, becoming a professor at the University of Hamburg. He also remained a physicist, conducting research for Germany's Max Planck Institute.

A dedicated pacifist, he continually pressed for scientists to take moral responsibility for the results of their work. He wrote several books, translated into more than a dozen languages, that analyzed the dangers of war in the contemporary world.

He is survived by four children. - AP