Who killed Russia opposition politician Boris Nemtsov?

In the absence of any arrest or claim of responsibility, there are multiple theories for the murder of Russian opposition politician Boris Nemtsov.

Was he killed in order to cow opposition to President Vladimir Putin at a time of mounting economic problems? Or because he opposed Russia's alleged covert war in Ukraine?

Or was he shot in full view of the Kremlin in an attempt to discredit Russia's leaders or even intimidate them, or incite a rebellion against them? Perhaps it was an opportunistic attack by someone harbouring a grudge?

Here are some of the theories circulating about who might be behind the killing and why. Some of them stretch the imagination, possibly in an attempt to obscure more obvious truths, bbc reports.

Early cordial relations between Boris Nemtsov (left) and Vladimir Putin, seen here at the Kremlin in July 2000, quickly deteriorated

Kremlin spokesman Dmitry Peskov has roundly dismissed the suggestion that the Russian leader could have ordered Nemtsov's killing, telling the BBC it was "illogical" and "unacceptable".

But Russia's best-known opposition leader, Alexei Navalny, has accused the Kremlin or lower officials of his friend's death.

Denied the right to attend Nemtsov's funeral as he sat in custody doing a 15-day sentence for illegal pamphleteering, he put out a statement (in Russian): "I believe that Nemtsov was murdered by members of a government (special services) or pro-government organisation on the order of the country's political leadership (including Vladimir Putin)."

The question, he added, was whether the order had been given to kill Nemtsov or to "stage an action that would have a high impact".

One of the puzzles of the Nemtsov assassination was that, as a liberal politician from the Yeltsin era, he did not enjoy the kind of popular support given to younger figures like Mr Navalny.

But Mr Navalny argued in his statement that Russia's leaders had decided to combat the country's developing economic problems with a crackdown on the opposition. It was no longer enough for them to "fabricate criminal cases", he said, in a clear reference to the cases brought against him since the mass election protests of 2011-12.

As for the perpetrators of the killing, he suggested they could have been members of informal militias allegedly used by the Kremlin as auxiliary police.

One argument that might counter his theory is that the security services already enjoy massive power under President Putin, whose own popularity rating increased last month to 86%, according to one poll.

At the time of his death, Nemtsov was organising an anti-war rally in Moscow and there is some suggestion that the Kremlin or rogue elements in the security forces might have acted to stifle dissent about the war.

"If you support stopping Russia's war with Ukraine, if you support stopping Putin's aggression, come to the Spring March in Maryino [a Moscow suburb] on 1 March," Nemtsov wrote in a social media post, published hours before he was shot (in Russian).

According to Ukrainian President Petro Poroshenko, Nemtsov was also planning to publish "some persuasive evidence about the involvement of Russian armed forces in Ukraine".

Although the anti-war rally was cancelled, Russia's small independent media have pressed on with investigating allegations of covert warfare.

It is impossible to know how real a threat Nemtsov posed to the authorities. And a ceasefire President Putin personally helped to negotiate for eastern Ukraine last month finally seems to be taking hold.

Foreign intelligence services

Mr Putin publicly condemned Nemtsov's murder, saying it was "entirely provocative in nature".

"Provocation" is Kremlin code for an attack aimed at destabilising the Russian state. As to who might be behind such an attack, the Kremlin's "chief spin doctor", TV anchorman Dmitry Kiselev, made clear who he thought stood to benefit most.

"When he was alive, Nemtsov was no longer necessary to the West, he had no prospects," he said. "But dead, he was a lot more interesting."

It is a standard line on Russian state-run media that the CIA orchestrated the uprising in Ukraine last year and anti-Russian unrest in other ex-Soviet states in recent years.

However, in the absence of any evidence, few people would take seriously the idea that the CIA staged the killing of Nemtsov in the hope of sparking a violent opposition reaction to President Putin's rule.

An alternative Russian theory, set out in the pro-government Izvestia newspaper (in Russian) which quoted an unnamed police source, is that the Ukrainian secret service had Nemtsov assassinated by Chechen hit men in order to destabilise Russia.

If this really was a genuine attempt to destabilise Russia, it may be too early to judge. So far opposition reaction has been largely muted, other than that of Mr Navalny in his jail cell.