Beijing resident Yang Jia, who allegedly murdered six policemen and injured four in a knife attack two weeks ago in Shanghai, could have been suffering from "temporary mental disorder", two lawyers offering to defend him said.

Beijing-based lawyer Xiong Liesuo questioned the objectivity of a Shanghai court statement released less than a week after the July 1 knife attack. It said the 28-year-old was "sober-minded and should therefore be held fully responsible for his behavior".

"Such an investigation normally takes about a month before any judgment is made," Xiong said.