Wang Lijun, the police chief of Chongqing city till last year, told the court in Jinan conducting the trial of Bo that Gu informed him "about murdering" British national Neil Heywood on November 14, 2011.
Gu was sentenced to death with a two-year reprieve earlier and Wang himself was sentenced to 15 years in prison for defecting to the US Consulate in Chengdu.
Two of Wang's proteges in the police force, Wang Zhi and Wang Pengfei, had already been "investigated and hushed", Wang said. The two officers were involved in the investigation of Heywood's death.
Wang Lijun said he learned that Wang Zhi, one of the officers, was summoned to Bo's home where senior police and city officials pressured him into writing a document saying that Gu was not involved in Heywood's murder, court transcripts released on its microblog account said.
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Bo abused his power when it was reported that Gu was suspected of intentional homicide, and when Wang Lijun defected to the US Consulate, state-run Xinhua news agency quoted the indictment as saying.
Facing examinations by prosecutors, Bo frequently said, "[I] cannot remember clearly. Prosecutors' investigation and verifications have final say."
Prosecutors, Bo and his lawyers also cross-examined Wang Lijun for about one hour and 40 minutes.
Wang proved all the facts about Bo's alleged abuse of power, Xinhua reported.
Prosecutors and the defending side also fully examined the documentary evidence and other witnesses' testimonies.