CHENGDU, China — A former Chinese police chief helped to cover up the murder of a British businessman by the wife of Bo Xilai, the Communist official toppled from power this year, but he also secretly collected evidence used to convict her, according to a lawyer for the police chief and an official account released Tuesday at the end of his trial.

During the trial, the former police chief, Wang Lijun, confessed to the most serious charge against him, defection, and did not contest the other three charges — abuse of power, taking bribes and bending the law for personal gain, a court spokesman said. The first two charges were heard in a closed-door session Monday because they involved state secrets, said the spokesman, Yang Yuquan, while the second two were reviewed Tuesday in a so-called public hearing from which journalists were barred.

It was unclear when a formal verdict or sentence would be announced. Like the trial of Mr. Bo’s wife, Gu Kailai, who was convicted last month of the murder of the Briton, Neil Heywood, the opacity of Mr. Wang’s trial raised questions about the fairness of China’s legal process. Verdicts in Chinese trials are often predetermined, especially when the trial is tied to elite politics.

Mr. Wang helped catalyze the downfall of Mr. Bo, and the biggest eruption in Chinese politics in a generation, when he fled to the United States Consulate in Chengdu, the capital of the southwestern province of Sichuan, in February and told diplomats there about Mr. Heywood’s murder last year. Mr. Wang, 52, had been the flamboyant police chief of the nearby city of Chongqing, which Mr. Bo ruled until his removal as party secretary this spring.