Nemtsov murder: Two held over killing of opposition politician Published duration 7 March 2015

media caption Sarah Rainsford reports from Moscow: ''The Kremlin wants to be seen to be investigating this case as quickly and as well as possible''

Two men have been detained over the murder of Russian opposition politician Boris Nemtsov, the head of the Federal Security Service (FSB) has announced.

Anzor Gubashev and Zaur Dadayev, both from the Caucasus region, were detained on Saturday, FSB director Alexander Bortnikov said.

Russian investigators said they were suspected of organising and carrying out the murder.

The killing on a bridge within sight of the Kremlin shocked Russia.

The former deputy prime minister and veteran liberal politician, 55, was shot in the back four times on the night of 27 February as he was walking on the street with his girlfriend. He was buried in Moscow on Tuesday.

Mr Bortnikov gave no details of how and where the suspects were detained, in the brief statement carried by Russia's state-owned Channel One, but he did say the investigation was continuing.

Neither of the men has yet been formally charged.

Analysis: Sarah Rainsford, BBC News, Moscow

The statement on the detentions was made by the head of the FSB, on Russia's main state TV channel. The message was clear: we are taking this crime seriously.

President Putin had ordered those responsible for what he called the "shameful and tragic" murder to be found. After all, it happened right next to the Kremlin, surely one of the safest spots in the country. He initially called that a clear "provocation".

But there is still very limited information on the suspects or their possible motives. If they are to be charged, that must be done in court. Russian media reports suggest the men were located using CCTV footage and phone records, as well as the testimony of "security officers" who were on the scene "by chance".

But in the absence of details, the reaction from friends and colleagues of Boris Nemtsov has been cautious so far. Above all, they want to know that the right suspects have been apprehended. Then they'll be asking whether they were contracted by someone to assassinate the politician, and why.

A source said to be familiar with the investigation told Russia's Interfax news agency the car allegedly used by the suspects was found rapidly and evidence gathered in the vehicle was used along with phone records to identify them, the source said.

Footage from security cameras had provided "sufficiently clear" images of the suspects, he added.

President Putin has publicly condemned the murder and called for an end to "shameful" political killings in Russia.

But leading opposition figure Alexei Navalny accused the Kremlin of ordering the assassination in order to cow the opposition amid Russia's mounting economic problems.

Russian investigators have publicly offered multiple motives ranging from Nemtsov's business dealings to his condemnation of the Charlie Hebdo attack in Paris.

Nemtsov was killed just days before a march he was organising against the war in Ukraine.

media caption The head of the security service, Alexander Bortnikov, made the announcement on Russian state television: ''Two suspects were detained today''

He had also been drafting a report expected to expose covert Russian military involvement in the conflict.

Some 50,000 people attended a rally near the Kremlin in protest at his death on 1 March and thousands turned out to pay their respects on the day of his funeral.

One Russian MP, Nikolai Kovalyov, told Ria-Novosti news agency that the key thing was to "find out who ordered this assassination".

Opposition activist Ilya Yashin said it was not enough to establish that the suspects had killed Nemtsov.

Writing on Facebook (in Russian), he said investigators needed to identify and arrest those who had ordered the murder.

"If those who ordered the murder are able to escape responsibility, the practice of political murders will definitely go on," he said.

image copyright Reuters image caption Flowers have been heaped on the spot where Nemtsov was shot