Those witnesses “are not mysterious and they are not new,” Mr. Dietrich said, referring to the men. “They gave statements, but these have not been correctly assessed.”

Image Gundolf Köhler Credit... via Associated Press

He declined to identify the new witness, or to give any more details about her, saying that she wanted her name kept private.

Mr. Dietrich said that he had talked with the woman by phone, and that he had written down what she said. After she signed a statement, he contacted the federal prosecutor’s office to ask that the investigation be reopened, he said.

An investigator then questioned the woman, Mr. Dietrich said.

The federal prosecutor, Harald Range, provided little of this detail when announcing on Thursday that the investigation would be reopened.

“There is no statute of limitations on murder,” Mr. Range said. “The statement by a woman witness who was not previously known prompted me to reopen formal inquiries into the Oktoberfest attack on Sept. 26, 1980. We will not restrict ourselves to this woman witness and her statements alone. Indeed, we will again comprehensively pursue all clues to clear up the background of this vile murder.”

Karl Rebmann, the federal prosecutor responsible at the time for investigating the bombing, examined the case for more than two years but failed to turn up evidence that Mr. Köhler had any accomplices in making or placing the bomb, or connections to any people or groups with a known history of such attacks.

The bomber was said at the time to have connections to right-wing extremists in the banned neo-Nazi Defense Sports Group, six of whose members were briefly detained after the bombing. Nothing was ever proved, and the investigation was stopped in November 1982.