All five appeared in a Moscow court and were detained as officials questioned them about the February 27 shooting death of Nemtsov, a staunch foe of Russian President Vladimir Putin. Nemtsov was shot four times in the back as he walked across a bridge with his girlfriend in sight of the Kremlin.

　　Judge Nataliya Mushnikova said one of the men charged with murder in the case, former deputy Chechen police commander Zaur Dadayev, signed a confession although no details were disclosed about his role. But the judge said a second suspect charged with murder in Nemtsov's death, Anzor Gubashev, denied his involvement.

　　Russian Judge: Chechen Confesses to Nemtsov Killing

　　Television footage showed Gubashev, who worked for a private security firm in Moscow, sitting in a jail cell holding a piece of paper over his face to hide it. Other images showed heavily armed police quickly escorting three hunched-over suspects into a small courtroom and locking them in barred cages.

　　Besides Dadayev and Gubashev, authorities identified the other three suspects as Gubashev's younger brother Shagid, Ramsat Bakhayev and Tamerlan Eskerkhanov.

　　Meanwhile, authorities said a sixth suspect blew himself up with a grenade as police tried to detain him in the Chechen capital, Grozny.

　　Before the judge disclosed Dadayev's involvement in the killing, his mother, Aaimani Dadayeva, told the Interfax news agency, "I can't believe it. He could not have committed this crime." She said her son has worked for Chechen law enforcement agencies for the last decade.

　　Motive still unknown

　　Despite the detention of the five suspects, no information has emerged about a possible motive for the killing. Nemtsov's allies say his assassination was ordered at the highest levels of the Russian government to silence critics.

　　All five suspects are from the restive North Caucasus region where Russia has fought two fierce wars in the last 20 years against separatists in Chechnya allied with Islamic fundamentalists. Security forces continue to clash with insurgents.

　　Putin has called the killing "a provocation" and vowed the government would do everything to ensure those responsible are "properly punished."

　　One of Nemtsov's closest allies in the opposition, Ilya Yashin, expressed his skepticism on Facebook after the arrests were announced, saying it is "hard to judge" whether these are the real perpetrators. He said it is extremely important that in addition to the shooters, those who ordered the killing are also identified and detained.

　　Nemtsov's killing occurred two days before he was to have led an anti-war and anti-Putin rally in Moscow. The opposition figure had also been working on a report about Russian military involvement in the Ukraine conflict.

　　After his death, the planned rally was transformed into a tribute to Nemtsov.

　　A few weeks before the shooting, Nemtsov told the Russian news website Sobesednik he thought Putin wanted him dead, and did not hold back his contempt for the Russian leader.

　　"I'm afraid Putin will kill me. I believe that he was the one who unleashed the war in Ukraine. I couldn't dislike him more," Nemtsov said.

　　Nemtsov was a deputy prime minister in the 1990s, and many Russian observers predicted he would succeed then-President Boris Yeltsin.

　　After President Yeltsin chose Vladimir Putin as his successor, and Putin was subsequently elected in 2000, Nemtsov became one of Russia's sharpest and most outspoken Putin critics, especially following last year's uprising in Ukraine.

　　In September, Nemtsov told VOA that Putin wants revenge for Ukraine's overthrow of its pro-Russian president.

　　He said Putin fears that what happened in Ukraine could happen in Russia, and sees a pro-European Ukraine as a threat to his own power.