It looked like one of the speediest murder investigations in Russian history. Nine days after the assassination of Russian opposition leader Boris Nemtsov, a lead suspect had been detained and had reportedly confessed to the crime.

But now the Chechen security officer accused of killing Nemtsov has claimed that law enforcement authorities forced him to confess.

Nemtsov, a prominent critic of President Vladimir Putin who was working on a report detailing Russian military support for rebels in eastern Ukraine, was killed by four shots to the back while strolling with his girlfriend some 300 feet from the Kremlin.

Zaur Dadayev, the recently dismissed deputy commander of an interior ministry battalion in the Russian republic of Chechnya, was arrested in the neighboring Ingushetia region and brought to Moscow, where he was charged for his alleged involvement in the crime on Sunday.

A judge said at the hearing in Moscow that Dadayev had confessed his guilt. But on Tuesday Dadayev professed his innocence to members of a prisoners' rights monitoring group in Lefortovo detention center, and said that he had been pressured to confess. He described being held in custody for two days with his hands and feet in shackles and a yellow bag over his head, according to a member of the monitoring group who also writes for the Moskovsky Komsomolets newspaper.

"They were yelling the whole time, 'You killed Nemtsov?' I was answering no," Dadayev said. "When I was detained, I was with a friend, my former subordinate Ruslan Yusupov, and they said that if I agreed they would let him go. I agreed. I thought that I would save him, and they would take me to Moscow alive. Otherwise what happened to Shavanov would have happened to me."

Beslan Shavanov, another member of the North battalion that is suspected of involvement in the murder, was killed in the Chechen capital of Grozny when police attempted to detain him on Saturday. Law enforcement sources reported that he had blown himself up with a grenade.

Four other suspects from Russia's Caucasus were detained besides Dadayev. One of them, Anzor Gubashev, has also been charged with the murder.

The prisoners' rights monitors described cuts and bruises on Gubashev's arms and legs. His brother Shagid, who is also a suspect, told them that police officers had beaten him.

"I thought they would take me to Moscow, and here I would tell the court the whole truth, that I'm not guilty. But the judge didn't even let me speak," Dadayev said. In court, Dadayev pointed one finger at the heavens in a common Muslim sign of faith. His only remark to reporters was, "I love the Prophet Muhammad." According to the monitors, he would have had to make a motion in court to speak, but the hearing was only to determine a period of arrest for the suspects.

The accused killer added that he had asked relatives to arrange a lawyer for him but had not heard back. Dadayev's family hired a lawyer named Zaurbek Sadakhanov to defend him, but then mysteriously dismissed him the next day. Sadakhanov has said that he was not allowed to visit his then-client in detention, and that Dadayev's testimony to investigators was therefore not legally obtained.

After Moskovsky Komsomolets published this account, Russia's investigative committee said in a statement on Wednesday that the prisoners' rights monitors who had visited Dadayev had violated the law when they "started to ask about materials of the criminal case." It noted that they would be interrogated to "find out what their motives were in asking about the details of the criminal case and publishing them in the mass media."

The North battalion is an elite counter-terrorism unit of 700 men that the Russian news site Gazeta.ru has reported "is considered part of the personal guard of Ramzan Kadyrov," Chechnya's authoritarian leader. It is commanded by the brother of Duma deputy Adam Delimkhanov, a cousin of Kadyrov whom the Chechen leader has called his successor.

Investigators have reportedly been coming to accept the theory that Dadayev organized the crime himself. In an Instagram post, Kadyrov said Dadayev had been "shocked" by support for the satirical Charlie Hebdo magazine in Paris, which was attacked by Islamic radicals in January over its caricatures of the Prophet Muhammad. Rosbalt news agency quoted law enforcement forces as saying that Dadayev had confessed that he had planned the murder himself in revenge for statements he said Nemtsov had made against Muslims.

But few experts believe that Dadayev decided on his own to coordinate a hit on Nemtsov. Gregory Shvedov, editor of the news site Caucasian Knot, told VICE News that Chechnya's higher leadership controls the movements of its men, making it unlikely that Dadayev traveled to Moscow and killed a prominent dissident near the Kremlin without his superiors' knowledge.

The esteemed independent newspaper Novaya Gazeta reported on Tuesday that Putin was informed last week of a Chechen "hit list" that included Nemtsov's name. The Russian president was reportedly told that a former North battalion officer who was related to a high-ranking Chechen official had ordered the hit. Novaya Gazeta identified the officer only by the name "Ruslan."

Russian anti-corruption activist Alexei Navalny tweeted that the man was likely Ruslan Geremeyev and that the influential relative in question was Senator Suleiman Geremeyev, a cousin of Delimkhanov.

Novaya Gazeta suggested that Nemtsov might have been killed as a stand-in for another opposition leader on the list: exiled Putin foe Mikhail Khodorkovsky. After Khodorkovsky spoke out in support of Charlie Hebdo, Kadyrov declared that he was an "enemy of all Muslims in the world, which means he's also my personal enemy."

Shvedov speculated that Nemtsov might have been killed by Chechen officials hoping to please Kadyrov.

As in the cases of various other killings of regime critics suspected of being politically motivated, Nemtsov's associate Ilya Yashin said that he fears that officials who might have helped plot the dissident's murder may never be brought to justice.

Putin's bestowal of a medal of honor to Kadyrov was interpreted by many political analysts as a signal that Kadyrov was not to be touched in the course of the Nemtsov investigation.