MOSCOW - Two men were detained in connection with last month's killing of opposition leader Boris Nemtsov, the head of Russia's federal security service said Saturday. But the identification of suspects did little to quell questions about the motive for the crime.

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

But authorities have released few details about the men or why they allegedly shot one of Russia's most prominent Kremlin critics.

Nemtsov, 55, was walking on a bridge near the Kremlin Feb. 27 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 into a car. Interfax cited an unidentified source Saturday as 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 suspicion that the Kremlin was involved in the attack.

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, Ukrainian model Anna Duritskaya, 23, 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.

"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.

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.