Russian authorities said on Sunday they were holding five men over the killing of Kremlin critic Boris Nemtsov, one of whom served in a police unit in the Russian region of Chechnya, according to a law enforcement official.

The five men were frogmarched into a Moscow courtroom on Sunday, forced by masked security officers gripping their bound arms to walk doubled over, a Reuters reporter at the court said. They stood in metal cages in the courtroom as television crews were ushered in to film them.

A judge ruled that all five should be held in custody and said that one of them, Zaur Dadayev, had admitted his involvement in the killing when questioned by investigators.

Dadayev served for a decade in the "Sever" police battalion, part of the interior ministry in Chechnya, Russian news agencies reported. They cited Albert Barakhayev, a senior security official in the neighboring region of Ingushetia, where several of the men were detained.

Nemtsov was shot dead on the night of Feb. 27 within sight of the Kremlin walls, in the most high-profile killing of an opposition figure in the 15 years that President Vladimir Putin has been in office.

Some associates of Nemtsov, a 55-year-old former deputy prime minister who became a Putin critic, say the Kremlin stands to gain from his death. Russian officials deny involvement and Putin has condemned the killing.

The court hearings on Sunday were given extensive coverage on state-controlled media, and presented as proof the authorities are conducting a thorough investigation - not the cover-up some of Nemtsov's friends say they anticipate.

But associates of Nemtsov say they will not be satisfied unless prosecutors track down whoever orchestrated the killing, rather than just the people who pulled the trigger.

There was no word from investigators on who the suspects were alleged to have been working for. The judge presiding over the hearings said investigators were still looking for others they believe were involved in the killing.

Russian media reports said most of those detained were from Chechnya or other parts of the North Caucasus, a poor and often violent area on Russia's southern flank.

Several other high-profile killings in Russia, including the 2006 shooting of journalist Anna Politkovskaya, have been attributed to gunmen from the North Caucasus, while those who ordered the crimes were never firmly identified.

CONFESSION

Judge Natalia Mushnikova told Dadayev's hearing at Moscow's Basmanny court: "Dadayev's involvement in committing this crime is confirmed by, apart from his own confession, the totality of evidence gathered as part of this criminal case."

Apart from Dadayev, investigators are holding two brothers, Anzor and Shagid Gubashev, and two others, Ramzan Bakhayev and Tamerlan Eskerkhanov. Previously, investigators said they only had two suspects in custody.

Two of the men, Dadayev and Anzor Gubashev, have been charged with involvement in the murder, while the other three are being treated as suspects.

Chechnya, a mainly Muslim region, has seen violent separatist insurgencies over the past two decades. It is now firmly under the control of its leader, Ramzan Kadyrov, a former rebel who changed sides and pledges loyalty to Putin.

There was no immediate confirmation from the Chechen authorities that Dadayev served in the "Sever" battalion, and it was not clear if Dadayev was still a serving member of it.

There have been cases in the past where employees of Russian law enforcement agencies have been prosecuted after moonlighting for organized crime groups.

Russia's Interfax news agency, quoting a Chechen law enforcement source, said a man killed in a standoff with police in the Chechen capital late on Saturday was wanted by police in connection with Nemtsov's killing.

The agency said when police arrived at an apartment block, the man threw one grenade at officers and then blew himself up with a second.

Nemtsov's closest aide told Reuters that the day before his death he clandestinely scribbled a note to her about how he was investigating the involvement of Russia's military in fighting in east Ukraine. [ID:nL5N0W750U]

Some of Nemtsov's friends have asked why the police took so long to arrive at the scene of the crime and how someone could fire six shots at him and get away in an area monitored by closed-circuit television footage. [ID:nL5N0W82XR]