West German police authorities said today that the left-wing terrorist Red Army Faction was responsible for both Tuesday's assassination attempt in Heidelberg against U.S. Gen. Frederick J. Kroesen and the bombing Aug. 31 at the Ramstein U.S. air base.

In a statement, investigators said that letters signed by the urban guerrilla group claiming responsibility for the two attacks were being taken as genuine. Further, the discovery last fall in a Heidelberg apartment of strategy papers outlining terrorist acts against U.S. personnel and facilities also confirm that the group was involved, the officials said.

The statement amounted to a formal recognition that the Red Army Faction is active again. It was responsible for bombings, kidnapings and killings in the mid-1970s but then disappeared from the scene after the loss of numerous leaders.

During a parliamentary debate today, Chancellor Helmut Schmidt again condemned the outbreak of anti-U.S. violence in his country, saying it threatened West Germany's own security and freedom. He also said President Reagan would visit West Germany later this year and would see that "Germans and Americans are friends."

Investigators said today they suspected that Christian Klar, 29, had taken part in the failed grenade and gunshot attack on Kroesen and might have been driving one of three vehicles thought to have been used in the ambush. Klar has been hunted for four years in connection with murders and kidnapings of prominent West Germans