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.
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.