Western leaders have called on the Russian authorities to get to the truth behind the killing of opposition politician Boris Nemtsov.
The US president urged a “transparent investigation”. The German chancellor said the killing must be explained and the perpetrators brought to justice.
Mr Nemtsov was shot four times in the back on a bridge near the Kremlin.
President Putin quickly condemned the killing and has taken personal control of the investigation.
Investigators say the murder may have been aimed at “destabilising” Russia.
Russian investigative committee head Vladimir Markin said in a statement that several motives for the killing were being considered including “Islamic extremism” and the victim’s alleged links with Ukraine.
“Mr Nemtsov may have been sacrificed by those who do not shun anything to reach their political gains,” the statement said.
It added that the attack was meticulously planned and the killers – who fled in a car – had been tracking Mr Nemtsov’s movements around the city.
Boris Nemtsov died hours after appealing for support for a march on Sunday in Moscow against the war in Ukraine.
Amid widespread global outrage, US President Barack Obama condemned the killing of Mr Nemtsov as a “brutal murder”.
Russian government must conduct a “prompt, impartial and transparent investigation”, the US president urged.
“I admired Nemtsov’s courageous dedication to the struggle against corruption in Russia and appreciated his willingness to share his candid views with me when we met in Moscow in 2009,” Mr Obama said in a statement.
A statement from the office of German Chancellor Angela Merkel condemned spoke of Mr Nemtsov’s “courage” for his frequent criticism of Russian government policy.
Mrs Merkel “calls on President Vladimir Putin to ensure that the murder is cleared up and the perpetrators brought to justice,” her spokesman Steffen Seibert said.
UK Prime Minister David Cameron echoed the calls for an inquiry, saying he was “shocked and sickened” by the news.
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Source: BBC