A former Chechen police officer was likely tortured into confessing to the murder of Russian opposition activist Boris Nemtsov, a member of the Kremlin's human rights council said on Wednesday, prompting a spat with investigators.
The statement from a body close to President Vladimir Putin and often criticised as window-dressing for authorities angered Russia's powerful Investigative Committee who accused it of meddling in their probe and threatened to investigate its motives.
Amnesty International said the allegations raised "very serious questions about the fairness of these proceedings and fuels speculation about a possible state-sanctioned cover-up."
Nemtsov, 55, a former deputy prime minister who became an outspoken Putin critic, was shot four times in the back as he walked along a bridge near the Kremlin on February 27 in the most shocking assassination of an opposition leader during Putin's 15 years in charge.
Investigators surprised by rounding up five suspects within a week and presenting them to a Moscow court.
Zaur Dadayev, a decorated former deputy commander of a police unit in troubled Chechnya, allegedly signed a confession and was charged with the murder on Sunday alongside Anzor Gubashev, who worked for a private security company.
The two men and three other suspects were remanded in custody.
However he has now insisted to the council's Andrei Babushkin that he is "innocent" and only made the admission under duress.
"There are reasons that lead us to believe Zaur Dadayev confessed under torture," Babushkin told AFP after a visit to the suspect's Moscow prison cell on Tuesday.
"We cannot confirm that he was tortured as we are not investigators but we did find numerous wounds on his body," said Babushkin.
The accusations were denied by a separate monitoring commission, which visited the prison on Wednesday and said there was no sign of bodily harm on the suspects.
"Zaur Dadayev says he was tortured, including with electric current, but the body parts he showed bore no evidence of any electrical current impact or burns," said Anton Tsvetkov, head of the Moscow Public Observer Commission.
'I am innocent'
Babushkin said Dadayev claimed he had made the confession under duress after being arrested in the North Caucasus region of Ingushetia last week, alleging that he spent two days manacled and with a sack over his head.
"They shouted at me all the time, 'You killed Nemtsov, didn't you?'. I said, 'no'," Babushkin reported Dadayev as saying.
The suspect said he had eventually admitted to the killing to secure the release of an ex-colleague, Ruslan Yusupov, who was detained alongside him.
"They said that if I confessed they would let him go. I agreed," Dadayev was quoted as saying by Babushkin.
"I thought that I would be brought to Moscow and would be able to tell the court the full truth. That I am innocent," Dadayev reportedly said.
Rights groups have consistently condemned widespread abuses in Russia's notorious prison system and during interrogations by officials.
Russia's powerful investigative committee condemned Babushkin's statement as breaking the law and said that he and a colleague who visited Dadayev would be questioned over possible "interference in a criminal case".
"They will be subject to an investigation aimed at clarifying their motives," investigators said in a statement.
Babushkin is one of some 60 unpaid members of the Kremlin council that includes writers, scientists, journalists and rights activists. The group meets Putin once a year and has been previously criticised as mere window-dressing for the authorities.
Council chief and presidential adviser Mikhail Fedotov describes his job as "the rock of Sisyphus".
In Greek mythology, Sisyphus was condemned to eternally rolling a boulder up a hill only to have it roll back once it was tantalisingly close to the top.
Nonetheless, Fedotov said he prided himself on several victories such as the release of the jailed Pussy Riot punk rockers.
Nemtsov's murder sent shockwaves through the opposition, which accuses Putin of steadily suppressing dissent and alleges that the Kremlin was behind the murder of one of its last outspoken critics.
Investigators have said they were looking into the possibility that the former deputy prime minister was killed over his support for French satirical magazine Charlie Hebdo, which published images of the Prophet Mohammed and was attacked in January.
But close allies of the prominent activist in Russia's marginalised opposition dismissed that claim as "absurd".
Authorities have also said they are probing the possibility that Nemtsov was murdered for criticising Russia's role in the Ukraine conflict or as part of a plot to destabilise the country.
Opposition newspaper Novaya Gazeta on Wednesday published a report into the investigation alleging that Nemtsov's killing was organised in Chechnya and that the Kremlin had uncovered a "hit list" listing the names of other Russian opposition figures, including former business tycoon Mikhail Khodorkovsky.
Kremlin spokesman Dmitry Peskov ridiculed claims that top officials had seen a list of any potential targets.
The press secretary for Khodorkovsky -- who currently lives in exile in Switzerland -- said Wednesday that she had found a mourning wreath propped outside the door of her Moscow apartment.