BERLIN (AP) — A 57-year-old German man was convicted Wednesday of killing the son of Germany's former president Richard von Weizsaecker and sentenced to 12 years imprisonment. He was ordered held in a psychiatric ward.

Fritz von Weizsaecker, a doctor, was stabbed to death in November while delivering a lecture on liver disease at a clinic in Berlin. An off-duty police officer who tried to stop the attack was seriously injured.

Berlin's regional court convicted the defendant, identified only as Gregor S. due to privacy rules, of murder and attempted murder.

Prosecutors said the defendant had acted out of hatred for the former German president, who died in 2015, blaming him for the use of the chemical known as “Agent Orange” during the Vietnam War.

Richard von Weizsäcker once worked for a German chemical company the killer believed had helped produced the defoliant widely used by U.S. troops during the conflict. The chemical has been blamed for health problems among those exposed to it.

Prosecutors, who had sought a 14-year prison sentence, said the defendant wanted “revenge” against the von Weizsaecker family.