Saeed Emami, the former Iranian deputy intelligence minister accused of masterminding the murders, committed suicide by swallowing hair remover while having a bath in prison, according to the official news agency. In January the government said that Mr Emami had organised the stabbing or strangling to death of three intellectuals and a husband and wife, who belonged to a minor political party, at the end of 1998.
The Iranian intelligence ministry is a bastion of opposition to reforms proposed by the country's President, Mohammad Khatami. His supporters have expressed doubts about Mr Emami's suicide, saying that prisoners in solitary confinement are inspected by guards every few minutes and are only allowed a five-minute bath once a week.
Mohammed Atrianfal, a reformist member of Tehran city council, said that Mr Emami had carried information to his grave about who was behind the murders. He said: "The prison authorities should have exercised a closer watch on a person like this."
Mohammed Niyazi, the military prosecutor, said on Tehran radio yesterday that the case could still be investigated, and he blamed foreign powers for playing a role in the murders. He said the suicide of Mr Emami "will not mean that we have lost all our leads - we have evidence and confessions that foreign hands were involved in the killings".
Mr Niyazi, who is in charge of the investigation into the killing of the intellectuals, said: "The aim of the murders was to start infighting among different political groups in the country and to tarnish the image of the Islamic Republic."
He warned the Iranian press not to muddy the waters by speculating about the motive for the murders "otherwise first we will warn them and then take legal action against them".
Some Iranian newspapers have suggested that Mr Emami was silenced in order to prevent him implicating other intelligence officers.
The struggle between moderates and hardliners in Iran has already heated up this month, with the arrest of 13 Iranian Jews from Shiraz and Isfahan who are to stand trial accused of spying for the United States and Israel. Iranian Jews say they fear that the arrests will spread to Tehran.
Meanwhile in the northern city of Mashhad a man was given 20 lashes for wearing eye shadow and plucking his eyebrows.