Two out of five men suspected of killing Russian opposition leader Boris Nemtsov were formally charged in a Moscow court, with one allegedly admitting involvement. A hearing for remaining three suspects was underway. (Reuters)

The head of Russia’s federal service said Saturday that suspects had been detained in connection with last week’s killing of opposition leader Boris Nemtsov, but the announcement did little to quell lingering questions about the motive for the crime.

Alexander Bortnikov said on Russian state television that two men — Anzor Kubashev and Zaur Dadayev, who are residents of the North Caucasus — were detained in the criminal case concerning Nemtsov’s death. Russian investigative committee spokesman Vladimir Markin later told the Russian news service Interfax that the two were “responsible for the organization and perpetration of Nemtsov’s killing.”

Russian state news agency RIA Novosti later cited Albert Barakhoyev, secretary of the Security Council of Russian republic of Ingushetia, located in the North Caucasus, saying that two more individuals had been detained, including Kubashev’s younger brother. Barakhoyev told RIA Novosti the detainees were ethnic Chechens.

But authorities have released few details about the men they say were responsible for Nemtsov’s death or why they allegedly shot one of Russia’s most prominent Kremlin critics.

Nemtsov, 55, was walking on a bridge Feb. 27 near the Kremlin with a female companion when he was shot by an unidentified gunman. Russia’s Interior Ministry said Nemtsov was killed by four shots to the back. Surveillance footage broadcast on a Moscow television station the next day showed the assailants disappearing in a car. Interfax cited an unidentified source Saturday saying that the car was identified quickly and that “biological materials” in it helped lead investigators to the suspects.

The federal security service has suggested no motive for the attack on Nemtsov, just days before he was scheduled to lead an “anti-crisis” march in Russia. But several theories have been circulating.

Some members of the Russian opposition have voiced the suspicion that the Kremlin was involved in the attack, which happened in one of the most heavily policed areas in the country. It took 11 minutes for an emergency vehicle to arrive on the scene.

Kremlin officials have framed the assassination as a “provocation” to discredit Russian President Vladimir Putin and foment social discord.

Russian officials have also floated several theories for the killing that ignore Nemtsov’s political differences with the Kremlin.

Russia’s investigative committee has said it is looking into possible connections between Nemtsov’s death and Islamic extremism, the Ukrainian conflict, Nemtsov’s condemnation of the Charlie Hebdo killings in Paris and his many personal and business relationships. It said it would also consider the possibility that Nemtsov had been a “sacrificial victim” — a none-too-subtle suggestion, akin to the Kremlin’s, that one of his allies killed him to smear the Kremlin.

A report in the Russian tabloid LifeNews even suggested Nemtsov’s murder might have been connected to a recent alleged abortion his companion had sought.

The companion, 23-year-old Ukrainian model Anna Duritskaya, told the Russian independent television station Dozhd that she saw neither the gunman nor the make, model or plates of the car that approached them.

Nemtsov’s allies have expressed doubt that those responsible for his death will be brought to justice, but some offered cautious hope Saturday that the detentions would lead to positive developments in the case.

“We hope that they detained those who really are related to the murder, that it is not a mistake,” Ilya Yashin, one of Nemtsov’s closest allies in the opposition, told Interfax. But he added that it is “difficult to judge” the actions of law-enforcement officials since they were not providing Nemtsov’s supporters with any special information on the progress of the investigation.

This is not the first time a high-profile assassination of a Kremlin critic has resulted in the detention of suspects from the North Caucasus, a volatile region straddling the border of Europe and Asia. The region includes Chechnya, where rebels have fought two wars against Russia in the past 20 years and which is now led by the Kremlin-backed autocrat Ramzan Kadyrov.

People from the North Caucasus have been named as suspects in the 2004 murder of Forbes editor Paul Klebnikov in Moscow, and five others were sentenced last year for the 2006 murder of journalist Anna Politkovskaya.

As one of the country’s most prominent Kremlin critics, Nemtsov belonged to a group that has felt increasingly targeted by Putin’s rhetoric. Many attribute his killing to the toxic atmosphere that has developed since Putin warned a year ago of “a fifth column” and “national traitors” undermining Russia from within.

Nemtsov had told Russia’s Sobesednik news site last month that he and his relatives feared Putin might try to have him killed because of his efforts to prove Russia’s responsibility for the war in eastern Ukraine. After his assassination, Ukrainian President Petro Poroshenko surmised that that was the reason for it.

Russian officials have denied that Russia is involved in the Ukrainian conflict between pro-government forces and pro-Russian separatists.

Last week, Putin called Nemtsov’s slaying a “disgrace,” adding that Russia had to rid itself of high-profile crimes and political murders.