A stabbing spree that killed six police and injured another four in Shanghai last week was revenge for a lengthy interrogation last year, the local police authority said on Monday.

Yang Jia, an unemployed 28-year-old from Beijing, was apprehended by police at 8:30 p.m. on Oct. 5, a Shanghai Municipal Public Security Bureau official said. He was then interrogated for six hours for riding an unregistered bike in the city's Zhabei District.

He was taken in after he refused to show his identification card and bike registration. Yang was released at 2:30 a.m. after the police confirmed the bike had been rented by him.

A torrent of e-mails sent later show Yang was disgruntled about the treatment he had received. He asked the authority to dismiss the relevant policemen involved and compensate him, the official said.

While he did receive replies to his letters from police, he felt they were unsuitable. In addition, Shanghai police traveled to Beijing twice in October and March to console the man.

On July 1, Yang stabbed a security guard at a police branch in Zhabei at 9:40 a.m.. After starting a fire at the facility gate, he then charged into the building and attacked nine officers with his knife. Six police died and another three police and a security guard were injured, the official said.

Yang was confirmed to be of a sane mind after a judicial appraisal following the attack.