A 20-year-old former policeman pleaded guilty Wednesday to fatally shooting his commanding officer at a police box in Shiga Prefecture in April, but sought leniency, with his lawyers citing his diminished capacity due to mental stress.

The defendant, whose name has been withheld because he was a minor at the time of the shooting, aged 19, admitted to killing police Sgt. Akira Imoto, 41, at the first hearing of his trial at the Otsu District Court. Imoto was posthumously promoted to police inspector.

The accused's lawyers argued he was struggling with adjustment disorder due to psychological stress and his "capacity to judge between right and wrong and to control his behavior had been reduced prior to firing the handgun."