One of men charged with the murder of Russian opposition politician Boris Nemtsov confessed under duress, a member of Russia's human rights council says.
Wounds on Zaur Dadayev's body suggested he had been tortured, Andrei Babushkin said after visiting the suspect in prison on Tuesday.
Officials say the visit should have been to check confinement conditions only and have warned of interference.
Mr Nemtsov was killed on 27 February.
Mr Dadayev and Anzor Gubashev, who both come from Chechnya, were charged on Sunday with his killing. Three other men, including Mr Gubashev's brother Shagid, are being held in connection with the case.
After visiting the suspects in prison, Mr Babushkin said there were "reasons to believe Zaur Dadayev confessed under torture".
He said Mr Dadayev had shown him marks from handcuffs and ropes around his legs, and told him he had been tortured with electricity.
He called for "people not involved in the investigation" to look into the claim.
Russia's investigative committee said Mr Babushkin and a journalist accompanying him had been allowed to visit the prison to inspect the conditions the suspects were being held under.
But they went beyond their remit by inquiring about the criminal case, which violated "not only the established norms, but the law," a statement said.
Both Mr Babushkin and journalist Eva Merkacheva would be questioned by investigators, the committee said.
Mr Babushkin, a member of Russia's advisory human rights council, said that during the visit Mr Dadayev had showed him multiple injuries he had sustained after his capture in Ingushetia on Saturday.
- Zaur Dadayev: Served as deputy commander in Chechnya's North Battalion, part of the regional interior ministry. Chechnya's leader Ramzan Kadyrov described him as one of the unit's "most fearless and courageous members"
- Anzor Gubashev: Zaur Dadayev's cousin and the only other suspect to have been charged. Russia's state TV suggested he might have been the gunman in Mr Nemtsov's murder
- Shagid Gubashev: Anzor Gubashev's younger brother. He says they were detained after hearing of Mr Dadayev's arrest and travelling to Malgobek, a city in the republic of Ingushetia. He claims they are innocent.
- Khamzat Bakhayev and Tamerlan Eskerkhanov: Little is known about them but they were reportedly arrested in Moscow and deny any involvement
Mr Dadayev, a former policeman, said he had only confessed to make sure a friend who was arrested with him would be released.
He had planned to tell a court hearing on Sunday the truth, Mr Babushkin said, but was not given a chance to speak.
Meanwhile, Shagid Gubashev, who continues to maintain his innocence, told the prison visitors he was in Chechnya when he learned that his cousin, Mr Dadayev, had been detained in the neighbouring Russian republic of Ingushetia.
He said he and his brother immediately travelled there and were detained as soon as they arrived. It would not have made sense for them to go to Ingushetia if they were connected to the crime, he added.
He said he and his brother were beaten and had bags pulled over their heads, which were only taken off after arriving in Moscow.
Anzor Gubashev said he had "no complaints" about the way he had been treated.
Kremlin spokesman Dmitry Peskov dismissed reports on Wednesday that President Putin had been told who organised Mr Nemtsov's killing, and top Russian officials had seen a "hit list" of other potential targets.
Mr Nemtsov, a former deputy prime minister and veteran liberal politician, was shot dead while walking with his girlfriend near the Kremlin.
Russian investigators have yet to cite a motive for his killing.
Last year, Mr Nemtsov contacted the Russian authorities after receiving death threats on his Facebook page.
In his request for an investigation, a copy of which has now been published (in Russian) by The New Times magazine, he linked the threats to his position with the conflict in Ukraine.
Police turned down his request in September.
A friend of Mr Nemtsov, Ilya Ponomarev, who is a member of the Russian Duma, told the BBC on Wednesday he believed the government was behind the killing and that the detained men were "scapegoats".
Chechnya's leader Ramzan Kadyrov has claimed that as a devout Muslim, Mr Dadayev was angry at those who defended the depiction of the Prophet Muhammad in the French satirical magazine Charlie Hebdo.
Mr Nemtsov had condemned the murder of 12 people at the magazine's office by Islamist militants, but those close to him say he was not a prominent critic of radical Islamism and focused his criticism on President Putin.