Two suspects have been arrested over the killing of Boris Nemtsov, the Russian opposition activist who was shot to death near the Kremlin in an assassination that shocked the country, according to the state security agency.
The arrests come a week after Nemtsov, who was 55, was shot four times in the back as he strolled with his girlfriend along a bridge in the heart of the capital, near the presidency and the Red Square.
"Two suspects were arrested today in connection with this crime, they are Anzor Gubashev and Zaur Dadayev, and the head of state has been informed," Alexander Bortnikov, the head of the FSB federal security service, told state television.
He said the two men were from the Caucasus region.
"The necessary operational and investigative procedures are continuing," Bortinkov said.
The latest killing of a prominent government critic under Putin's rule prompted an outpouring of international condemnation and angered members of an opposition who blamed the Kremlin for encouraging hatred against anyone who expresses dissent by referring to them as "traitors".
Nemtsov, an anti-corruption crusader who served as Boris Yeltsin's first deputy prime minister in the 1990s, was killed just two days before he was to lead a major anti-government rally.
However the protest march - called to denounce Russia's alleged role in the Ukraine crisis - instead became a massive memorial for Nemtsov, with tens of thousands swarming the streets of Moscow in the largest opposition gathering since a wave of anti-Kremlin protests in 2011-12.
Putin had promised an all-out effort to catch those responsible for an act which he called a "provocation".
Theories have proliferated since the killing over why Nemtsov was targeted.
Some suggest he was assassinated for criticising Russia's role in the Ukraine conflict, others for his condemnation of January's killings at the Charlie Hebdo satirical weekly in Paris.
Friends said Nemtsov had been working on a report containing what he described as proof of Russian military involvement in the bloody uprising by pro-Moscow fighters in eastern Ukraine.
Meanwhile, investigators suggested the killers wanted to destabilise Russia, which is facing its worst standoff with the West since the Cold War over Ukraine, and Putin's allies hinted at a Western plot.
Following the announcement of the arrests, the former head of the FSB - the successor to the Soviet-era KGB - and now legislator Nikolay Kovalev told the RIA Novosti agency that initial information showed the two men were merely paid hitmen.
"The key is to find out who ordered this assassination," he said.
A fellow opposition activist, Ilya Yashin, welcomed the development in the case, but called for more information on the men's identities.
"We hope the arrest ... is not an error but the result of good work by security forces, but for now it is hard to say," Yashin told Interfax news agency.
"Quite frankly the execution of the investigation had not inspired any optimism, but the fact that there have been arrests inspires some optimism."
Nemtsov, a charismatic orator who was one of the last outspoken opponents to Putin, was a key speaker at mass opposition rallies against Putin's return to the Kremlin in 2012.
He wrote several reports critical of corruption and misspending under Putin.
In 2013, he said up to $30bn of the estimated $50bn earmarked for the Olympic Games that Russia was to host in Sochi had gone missing, which the Kremlin denied.
Source: Al Jazeera and agencies