MOSCOW — A member of the Kremlin’s advisory council on human rights said on Wednesday that the main suspect in the shooting death of a high-profile opposition figure was most likely forced to confess under duress, and that his two cousins in detention had been tortured.
After visiting the three Chechens, who were among five suspects imprisoned on Sunday in Lefortovo Prison in Moscow, Andrei Babushkin, a rights activist, said that the men had suffered multiple injuries after their arrest.
In a summary of the visit posted on the council’s website, Mr. Babushkin also reported that another man arrested at the same time as Zaur S. Dadayev, the main suspect, had disappeared and said that he had asked Russia’s top law enforcement agency to account for his whereabouts.
The report caused an immediate stir in the Russian government. The Investigative Committee, which is responsible for looking into the Feb. 27 killing of the opposition figure, Boris Y. Nemtsov, near the Kremlin, accused Mr. Babushkin and Eva Merkacheva, another rights official, of violating the law.
The statement issued by the committee questioned the motives of Mr. Babushkin and Ms. Merkacheva, hinting that they could face charges of trying to hinder the investigation of a crime, which carries a possible jail sentence of up to six months.
Mr. Dadayev, who is suspected of being the assassin according to Russian news reports, has “numerous wounds on his body,” Mr. Babushkin said. He added that Mr. Dadayev’s cousin, Anzor Gubashev, who is suspected of being the driver of the getaway vehicle, had cuts on his nose, wrists and legs.
Mr. Babushkin said Mr. Dadayev had told him that, after he was arrested on March 5, he was starved, given only sips of water three or four times a day and left hooded until he was transferred to Moscow for his court appearance on Sunday.
Mr. Gubashev had no complaints about his treatment, the report said, but his younger brother Shagid Gubashev, also in jail, told the two activists that his older brother had been beaten and pressured to confess. “There are reasonable grounds to believe that Dadayev and the Gubashevs were tortured,” Mr. Babushkin wrote.
According to the report, Mr. Dadayev said that another man detained with him, Yusupov Rustam, had later disappeared.
According to Mr. Babushkin, Mr. Dadayev, previously a lieutenant in the Interior Ministry’s forces in Chechnya and decorated for bravery, said that Mr. Rustam was his former subordinate and that he had confessed to killing Mr. Nemtsov because he was told Mr. Rustam would be released unharmed if he did.
It is Mr. Rustam’s whereabouts that the members of the rights council asked of the F.S.B., the Russian abbreviation for the Federal Security Service, previously the K.G.B.
Mr. Dadayev and Mr. Gubashev, both charged in the murder, were ordered on Sunday to be jailed until April 28. The other three suspects, including Mr. Gubashev’s younger brother, were not formally charged but were jailed pending further investigation.
Ramzan A. Kadyrov, the leader of Chechnya, has said he knew Mr. Dadayev personally as a faithful Muslim. Mr. Kadyrov has suggested that the motive for the killing was Mr. Dadayev’s anger about Mr. Nemtsov’s defending the right of a French satirical weekly to publish cartoons mocking the Prophet Muhammad.
That motive was widely rejected by others who have linked the killing to Mr. Nemtsov’s criticism of Russia’s role in the war in Ukraine.