A South African magistrate ruled yesterday that no one bears criminal responsibility for the death of a young black man who plunged from a fifth-floor window while in the custody of security police last July.

The man, Lungile Tabalaza, 19, was in the same Port Elizabeth police station in which Black consciousness leader Steve Biko was being held when he was fatally injured in September 1977. Tabalaza was the 23rd person to die while in the custody of South African security police since 1976. Domestic and foreign critics of the South African government have questioned the circumstances of many of these deaths.

Magistrate J. A. Coetzee said he could not determine from the evidence why Tabalaza jumped from the window but said two policemen who had been questioning him could not be held responsible.

Their superior officer, Col. Piet Goosen, headed the police unit that was holding Biko and was a principal witness at the inquiry into Biko's death.

Despite government instructions that prisoners were to be interrogated only in rooms with barred windows, Tabalaza was questioned in a room with an open unbarred window and was said to have jumped moments after his questioners left him alone.

Police speculated that he was trying to escape to the roof of a nearby building, but a lawyer for his family argued that he jumped because of "what happened or what he feared would happen" in the room.