Russian officials investigating the murder of opposition politician Boris Nemtsov have claimed Islamist extremists may have been behind his death.
Other possible motives listed by the Investigative Committee, which has Vladimir Putin as its executive, included an attempt to destabilise Russia, the Ukraine conflict and his personal life.
Mr Nemtsov was shot four times in the back by a gunman in a car as he crossed a bridge in view of the Kremlin just before midnight on Friday, police said.
The 55-year-old was walking with a Ukrainian woman after they ate at a restaurant in Red Square. She escaped unhurt.
A statement from the Investigative Committee said Mr Nemtsov may have been a “sacrificial victim for those who do not shun any method for achieving their political goals”, echoing the comments by Mr Putin's spokesperson and other Russian politicians calling the attack a “provocation”.
Possible connections to the Ukrainian conflict and Islamic extremism are also being investigated, it said.
The Committee announced it was considering whether there was "personal enmity" towards the politician in his private life as state-controlled television gave considerable attention to the woman he was gunned down in front of, identifying her as a Ukrainian model 30 years his junior.
Officials did not address accusations by the former Deputy Prime Minister’s supporters that he was killed for being one of Mr Putin’s most prominent critics.
Speaking to Russia's Sobesednik news website on 10 February, Mr Nemtsov had said he feared for his life.
"I'm afraid Putin will kill me,” he added. “I believe that he was the one who unleashed the war in the Ukraine. I couldn't dislike him more."
Hours before his death, Mr Nemtsov denounced the President’s policies as "mad, aggressive and deadly" and was due to lead an anti-government Spring March on Sunday protesting against the Kremlin’s alleged involvement and the economic crisis in Russia.
He was also working on a report containing evidence that he believed proved Russia's direct involvement in the separatist rebellion that has raged in eastern Ukraine since April.
Moscow has continually denied accusations it is supporting the rebels with troops and sophisticated weapons.
Mr Putin condemned the murder of Mr Nemtsov, which his spokesperson Dmitry Peskov said could be a possible "provocation" coming just a day before the march.
Organisers have since cancelled the rally and instead called for a demonstration to mourn him on Sunday in central Moscow.
The city gave quick approval for that gathering, in contrast to its usual slow and grudging permission for opposition rallies.
Through Saturday morning, hundreds of people came to the site of Mr Nemtsov's death in the shadow of the Kremlin to lay flowers.
Mr Putin has ordered Russia's top law enforcement chiefs to personally oversee the investigation.
Britain, the US, Germany and other countries have all expressed sadness and called for a full and transparent inquiry to bring the perpetrators to justice.
David Cameron said he was "shocked and sickened" at the "callous murder".
"This despicable act must be fully, rapidly and transparently investigated, and those responsible brought to justice," the Prime Minister added.
"Boris Nemtsov was a man of courage and conviction. His life was dedicated to speaking up tirelessly for the Russian people, to demanding their right to democracy and liberty under the rule of law, and to an end to corruption.
"He did so without fear, and never gave in to intimidation. He was greatly admired in Britain, not least by his friend Lady Thatcher, who visited him in Russia and who would have been appalled by today’s news.
"The courage of Nemtsov’s life contrasts with the utter cowardice of his murder.
"I extend my condolences to Boris Nemtsov’s family and friends. The Russian people have been deprived of a champion of their rights. Boris Nemtsov is dead. But the values he stood for will never die."
Mr Nemtsov served as a regional governor and then Russia’s Deputy Prime Minister in the 1990s and once was seen as a possible successor to Boris Yeltsin, the first elected President.
After Mr Putin came to power in 2000, Nemtsov became one of his most vocal critics and fell out of favour with the Kremlin.
Additional reporting by PAReuse content