Zaur Dadaev, a former Chechen police officer who pleaded guilty on Sunday to killing Russian opposition leader Boris Nemtsov “likely confessed under torture,” a member of the Kremlin’s rights council told Agence France-Presse (AFP) on Wednesday. The 55-year-old former deputy prime minister, and a vocal critic of Russian President Vladimir Putin, was fatally shot on Feb. 27, near the Kremlin.

Dadaev was arrested with four other suspects in connection with Nemtsov’s murder, while a sixth suspect blew himself up with a grenade in Grozny, Chechnya, when law enforcement officials tried to take him into custody. Dadaev, who was the only one who has so far admitted to Nemtsov’s killing, was charged on Sunday along with Anzor Gubashev who reportedly worked for a private security company in Moscow.

"There are reasons that lead us to believe Zaur Dadayev confessed under torture," Andrei Babushkin told AFP, adding that he had seen "numerous wounds" on Dadaev’s body during a visit to his Moscow prison cell on Tuesday. Babushkin also said that Dadaev claimed that he had made the confession under duress after he was arrested in the North Caucasus region of Ingushetia.

Dadaev also alleged that he was manacled for two days and had a sack over his head, Babushkin told AFP, adding that the former eventually admitted to the killing to secure the release of an ex-colleague, Ruslan Yusupov, who was arrested along with him.

"They said that if I confessed they would let him go. I agreed. I thought I would save him and they would bring me to Moscow alive," Dadaev reportedly said, according to Babushkin.

Dadaev, the prime suspect in the case, is linked to controversial Chechen leader Ramzan Kadyrov, who is a strong supporter of Putin, and had reportedly praised Dadaev as a “true Russian patriot” for his military service.

Nemtsov was reportedly walking over the Great Moskvoretsky Bridge near the Kremlin in Moscow with his girlfriend, Anna Durytska, when a man rushed out of a car behind him and shot him four times. 

The motive behind Nemtsov's assassination remains unclear, but police suspect Dadaev and some of his accomplices were paid to carry out the hit in retaliation for Nemtsov’s criticism of Islamist gunmen involved in the attack on the French satirical magazine Charlie Hebdo in January. However, some of Nemtsov’s close allies believe the hit was orchestrated by high-level Russian officials for his criticism of Putin's administration.