It is unclear, however, whether Ms. Gu suspected Mr. Heywood of spying on her family. A lawyer for Ms. Gu’s family said it was never mentioned to him that she thought Mr. Heywood was a spy.

“I have never heard from Gu’s family or anyone else that Heywood had any connection with intelligence departments, as a source or a spy,” the lawyer, Li Xiaolin, said in an interview. “Nothing that links Heywood to intelligence work was presented at Gu’s trial either.”

Adding to questions about the importance of espionage in the case, friends said Mr. Heywood had become estranged from the powerful family in the year before his death in November 2011. He also openly cultivated the image of an inside operator, driving around Beijing in a sports car with “007” license plates. Mr. Heywood had done work for the private intelligence firm Hakluyt, founded by former officials with MI6, helping prepare due-diligence reports on Chinese companies for investors. That association had given rise to longstanding speculation that he was a spy.

Reports have varied as to the motives behind the murder of Mr. Heywood. Ms. Gu contended that she had been trying to protect her son. According to the testimony presented at her trial, Ms. Gu claimed that Mr. Heywood had threatened him over a bad real estate deal.

At her trial, according to state media reports, Ms. Gu admitted poisoning Mr. Heywood after he came to visit her in Chongqing, the city-state that her husband ruled. Mr. Heywood had vomited after drinking too much and requested water. Ms. Gu admitted pouring potassium cyanide down his throat.

Initially, the cause of Mr. Heywood’s death was listed as a heart attack; a murder investigation began after Mr. Bo’s police chief fled to the American Consulate in a nearby city early this year. He told American diplomats of his suspicions and provided evidence of Ms. Gu’s involvement in Mr. Heywood’s death. They informed the British authorities.

When the matter came to light on Chinese Web sites and in Western media reports, Mr. Bo’s opponents within the party hierarchy were able to force his dismissal from his posts and engineer his ouster from the Politburo. He had been considered a serious contender to win a position on the top-level Standing Committee at this week’s party congress.

He was recently expelled from the party and faces criminal prosecution on charges of corruption, abuse of power and tampering with the investigation into Mr. Heywood’s death. Ms. Gu and the police chief, Wang Lijun, have been sentenced to prison.