Two suspects have been detained over the killing a week ago of opposition politician Boris Nemtsov, the head of Russia's federal security service said today.
Alexander Bortnikov, in comments shown on state television, named them as Anzor Gubashev and Zaur Dadaev and said they were from Russia's North Caucasus region, but gave no further details.
He said they were "suspected of carrying out this crime", but it was not clear if either of the suspects was believed to have fired the shots that killed Mr Nemtsov as he and a companion walked over a bridge near the Kremlin in Moscow on February 28. No charges were immediately announced.
Mr Nemtsov's killing shocked Russia's already beleaguered and marginalised opposition supporters. Suspicion in the opposition is high that the killing was ordered by the Kremlin in retaliation for the 55-year-old's staunch criticism of President Vladimir Putin. Mr Nemtsov had been working on a report about Russian military involvement in the eastern Ukraine conflict.
But Russia's top investigative body said it was looking into several possible motives, including that Mr Nemtsov was killed in an attempt to smear Mr Putin's image. It said it was also examining possible connections to Islamic extremism and Mr Nemtsov's personal life.
Many believe that Mr Nemtsov's murder in a tightly secured area near the Kremlin would not have been possible without official involvement, and could be an attempt to scare other government opponents.
Mr Putin dubbed Mr Nemtsov's killing a "provocation".
One of Me Nemtsov's closest allies in the opposition, Ilya Yashin, said on Facebook after the announcement that "it's hard to judge whether these are the real performers of if the investigation went down a false track".
In any case, he said, "it's extremely important that the matter not be limited to detention of the shooters, whether these are the real killers or not. The key task is the identification and detention of who ordered" the attack.
"For the time being, it's very skimpy information, which tells us little, but it's good that the first results of the investigation has appeared," another opposition leader, former prime minister Mikhail Kasyanov, was quoted as telling the news agency Interfax.
In some previous killings of Kremlin critics, especially the murder of journalist Anna Politkovskaya in 2006, there has been wide criticism that those who ordered the killing have not been identified or prosecuted.
The North Caucasus region from which the suspects reportedly come includes Chechnya, where two separtatist rebels fought two wars against Russian forces over the past two decades and which now is in under the tight control of Kremlin-backed leader Ramzan Kadyrov.
Mr Kadyrov has been widely criticised for brutality against opponents, including summary executions and abductions, and is a vehement defender of Mr Putin. He blamed Western security services for Mr Nemtsov's killing.
No further information was given about Gubashev and Dadaev, but opposition figures unearthed a statement from the Chechen government from 2010 in which a Zaur Dadaev was among the police troops awarded medals.
Kremlin critics say the spiteful nationalist propaganda on state television, which cast Mr Nemtsov and other liberals as Western stooges, helped prepare the ground for his killing.
"The atmosphere of mad aggression created by the state television ... has signalled that you could do anything to the people expressing a different view, this will benefit the Motherland," Dmitry Muratov, the editor of the Novaya Gazeta, a newspaper critical of the Kremlin, told the Associated Press yesterday.
Mr Nemtsov was walking with a young Ukrainian woman, Anna Duritskaya, when he was shot.
She has returned to Ukraine after questioning by police, and today the state news agency, RIA Novosti, quoted her lawyer, Vadim Prokhorov, as saying she has not been called back to Russia to give evidence in connection with the detentions.