One of the men detained on suspicion of killing Russian opposition figure Boris Nemtsov served in a police unit in the Russian region of Chechnya, Russian news agencies quoted a law enforcement official as saying.
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.
Two buses arrived on Sunday at the Moscow court where the two men investigators say they have detained over the killing, Anzor Gubashev and Zaur Dadayev, were scheduled to appear before a judge to be formally arrested.
A Reuters reporter said the buses, with blue flashing lights and with a police escort, drove into the back entrance of the court. Several people wearing ski masks and in black clothes could be seen getting out.
It was not possible to see if the people being delivered were the two suspects, but large numbers of police lined the street outside the Basmanny district court, and guarded the entrances.
Dadayev served for around 10 years in the "Sever" battalion of Chechnya's interior ministry, Russian state-controlled news agencies quoted Albert Barakhayev, Security Council secretary in the neighbouring Ingushetia region, as saying.
Gubashev and Dadayev were detained on Saturday in Ingushetia, where some members of their families live, according to Barakhayev. He said a relative of Gubashev was also detained, though this has not been confirmed by officials investigating Nemtsov's killing.
It was not immediately clear if Dadayev is a serving member of the battalion, or has left. There was no confirmation from the authorities in Chechnya.
Chechnya is a mainly Muslim republic on Russia's southern border. It has been the scene of violent separatist insurgencies over the past two decades.
It is now firmly under the control of its leader, Ramzan Kadyrov, a former rebel who now pledges loyalty to Moscow and has considerable autonomy over the running of the region, including its security services.
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.
Supporters of Nemtsov, a 55-year-old former deputy prime minister, have said the Kremlin stood to benefit from his killing. Officials have denied involvement and Putin has called the shooting a shameful tragedy.
Nemtsov's associates say they will only be satisfied if prosecutors track down whoever orchestrated the killing, and not just the people who pulled the trigger.
Low-level foot soldiers -- in many cases from Chechnya and neighbouring regions -- have been put on trial in several previous high-profile killings in Russia, while it has never been firmly established who ordered the crimes.
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