The father of convicted police killer Yang Jia said yesterday that his son's mother had been committed to a mental hospital, explaining her disappearance for the past four months. Yang was sentenced to death for killing six officers in an attack on a Shanghai district police station on July 1. His mother did not attend the trial, which started in August, or an appeal last month. Her whereabouts were relevant because she had agreed to the court-appointed lawyer for the trial while out of contact with other relatives. Despite the violence of the crime, Yang gained sympathy as an alleged victim of police brutality, which he claimed drove him to take revenge. His father, Yang Fusheng , said his ex-wife and Yang Jia's mother, Wang Jingmei , was in Ankang Hospital in Beijing's Shunyi district. The hospital is under the Beijing Public Security Bureau. Mr Yang is divorced from her and estranged from his son. Mr Yang said that on November 8 an official told Yang Jia's aunt that she could visit her sister, and she did the next day. He said he was not told directly, but heard about the visit through another person. He said Ms Wang had told her visiting sister that she was taken to the hospital on July 2, the day after the attack, and then signed the letter granting power of attorney. There have been allegations that Yang's mother was 'kidnapped' to prevent her from being more involved in her son's defence. She reportedly tried to commit suicide. Yang's father opposed the original choice of lawyer, Xie Youming . Legal experts said he had a conflict of interest because the government of Zhabei district, where the attack took place, had been a previous client. On July 1, Yang Jia threw a handmade bomb at the gate of the district police station and stabbed a security guard before entering the building and stabbing nine officers. Prosecutors said he was stopped by police for riding an unlicensed bicycle the previous October and carried out the attack after failing to get compensation. Yang's lawyer for his appeal, Zhai Jian , said yesterday he was unaware of the latest developments. The case is awaiting review by the Supreme People's Court in Beijing, but another lawyer would handle the procedures, Mr Zhai said. 'I'm not the lawyer who is handling his death sentence review, so I don't know how it is going,' he said.