As mourners from around the world gather in Moscow for the funeral of murdered Russian politician Boris Nemtsov today, many unanswered questions still surround his death.
Four days after he was shot dead in the heart of the capital, no arrests have been made and no clear motive has been established.
What we know
Before the murder
Mr Nemtsov, 55, had dinner with his girlfriend, Ukrainian model Anna Durytska, in Red Square.
Shortly after 11pm, the couple headed towards the picturesque Bolshoy Moskvoretsky Bridge to cross over the Moskva, allegedly making their way towards the politician’s apartment.
How he died
They were walking hand in hand when a gunman shot Mr Nemtsov four times in the back with a pistol at around 11.30pm.
The killer then jumped over a barrier into the road and got into a waiting light-coloured car, which sped off.
Traffic was streaming over the busy bridge at the time of the shooting but the only witness to be quoted in Russian media is Miss Durytska, who was unharmed.
The 23-year-old said in her first interview that the killer shot the former Deputy Prime Minister from behind before jumping into a passing vehicle.
She did not see his face and could not remember the make or model of the getaway car.
A police source told the Russian Kommersant newspaper that old bullets were used.
The pistol was believed to be either a Makarov, the standard gun issued to police and soldiers in the Soviet era, or an IZh gas pistol modified to use live ammunition.
Police told Russian media outlets they were looking for a man around 5ft 7ins tall, of normal build and with dark cropped hair.
He was wearing blue jeans and a brown jumper.
What we don’t know
Who was behind it?
The Investigative Committee looking into Mr Nemtsov’s murder has listed a stream of possible suspects including Islamist extremists, ultra-nationalists, enemies from his personal life and groups who wanted revenge for his opposition to the Ukrainian conflict.
Outside Russia, the opposition politician’s vocal criticism of Vladimir Putin has put suspicion on the President but his spokesman said any suggestion of involvement was “illogical" and "unacceptable".
Speaking to Russia's Sobesednik news website on 10 February, Mr Nemtsov said: "I'm afraid Putin will kill me. I believe that he was the one who unleashed the war in the Ukraine. I couldn't dislike him more."
Analysts have questioned why Mr Putin would order the death of Mr Nemtsov while letting far more prominent critics, like Alexei Navalny, live.
Why was Mr Nemtsov killed?
He served as Russia’s Deputy Prime Minister in the 1990s and once was seen as a possible successor to Boris Yeltsin, the first elected President, but fell out of favour after Mr Putin came into power.
His criticism of the President, opposition to alleged Russian involvement in Ukraine and report on the subject, condemnation of the Charlie Hebdo massacre and “womanising” personal life have all been suggested as motives.
Mr Nemtsov had been involved in a well-publicised anti-corruption campaign in Yaroslavl, not far from Moscow, and had a number of business interests.
The Kremlin has suggested the murder may have been a “provocation” intended to put blame on the Government and destabilise the state.
Was the gunman a professional?
There have been differing opinions on whether the shooter was a contract killer, with some suggesting that the precision of the shots while leaving Ms Durytska unharmed suggested a high level of training.
But a police source told Russian daily Kommersant that old bullets were used, possibly fired from a homemade gun. “Participants in the investigation are only sure of one thing - that the killers were not professionals,” the source said.
Was the snow plough involved?
A snow clearing vehicle was crawling along the bridge at the time of the murder, obscuring Mr Nemtsov from view at the precise moment of his murder from the one CCTV camera that could see the bridge.
Observers have suggested the timing could be suspicious but the alleged driver told Russian television of his shock at the shooting, which he did not witness.
“I looked into the rear mirror and saw a man on the ground," he said. "I immediately realised he didn’t feel well so I drove a little bit further and pulled over.”
The man said he met Miss Durytska as she ran towards him, saying Mr Nemtsov had been shot, and he called emergency services.
Why was there no clear CCTV?
The poor quality, wide angle CCTV where the murder is obscured by the snow plough is the only known footage of Mr Nemtsov’'s death.
The lack of recordings in the heart of Moscow has sparked questions for authorities, who have claimed all cameras were working properly.
The Federal Guard Service, which controls the area around the Kremlin, said none of its surveillance cameras were pointed at the bridge and the area “is not part of its zone of responsibility”.
Moscow’s Department of Information Technologies, in charge of the surveillance cameras in pedestrian areas, also claimed to have no footage, passing responsibility back on to the Federal Guard Service in comments to RIA Novosti.
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