Russians were operating a BUK missile launcher seen in the area where the Malaysia Airlines passenger jet MH17 was shot down, eyewitnesses have told Panorama.
Distinctive shrapnel damage to the plane points to a fragmentation weapon - such as a BUK - downing the plane.
The Kremlin had previously suggested that the missile was from a Ukrainian fighter jet.
Pro-Russian rebels have denied any possession of a BUK.
However, photographs and videos cast doubt on the claim by the Kremlin and pro-Russian rebels - that they did not have a BUK missile launcher on their territory.
Three eyewitnesses, all civilians, separately told Panorama that they saw a missile-launcher in rebel-held territory a few hours before the Boeing jet was hit.
One eyewitness saw the missile-launcher roll off a low-loader at Snezhnoye, around ten miles from the crash site, at around 13:30 local time (10:30 GMT).
"We just saw it being offloaded and when the BUK started its engine the exhaust smoke filled the whole town square," he said.
'Pure Russian accents'
The eyewitness told the BBC that the crew struck him as Russian soldiers: "Well-disciplined, unlike the rebels, and not wearing the standard Ukrainian camouflage uniform sported by government and rebel troops alike."
"They had pure Russian accents. They say the letter 'g' differently to us," he said.
In eastern Ukraine, most people speak Russian but the BUK crew did not speak Russian with a local accent.
His testimony was confirmed by a second eyewitness, who added that an officer in a military jeep escorting the BUK spoke with a Muscovite accent.
If correct, the Kremlin has to explain to the relatives of the 298 passengers and crew who died - including ten Britons - why Russian military personnel were allegedly seen in the area escorting a BUK shortly before MH17 was shot down.
If these eyewitnesses are right, then the BUK crew may have been part of the Kremlin's 'Ghost Army' - reportedly thousands of Russian soldiers who have been secretly infiltrated into Ukraine and have tipped the military balance heavily in the rebels' favour.
The Kremlin's denial of military support to the rebels is countered by a mass of evidence, including a video of a mechanised battalion column of Russian tanks and military hardware shot from the back of a bus in Ukraine.
Fresh graves of alleged Russian soldiers killed in the fighting in Ukraine also tell a wholly different story to the Kremlin's official line that it has no role in the war.
Last week Panorama caught up with Mr Putin in Yakutsk's Mammoth Museum in Siberia and asked him if he regretted the killings in Ukraine.
He parried the question and was quick to blame the Ukrainian government for not talking to the rebels and its army for "shelling residential areas".
The president has a point. When I was in Donetsk in July, the Ukrainian army, seeking to shell a rebel base in the city, missed and hit the maternity hospital. Fortunately expectant mums and newborn babies had already been moved down to a cellar.
Human Rights Watch has condemned both the government and rebel sides for indiscriminate shelling of civilians.
But Mr Putin left as I tried to tackle him on Russia's actions in the war and MH17, so those questions remain unanswered.
Mikhail Khodorkovsky, once Russia's richest man who spent ten years in prison after he crossed President Putin, said:
"The prospect of democratic change of power in Russia does not exist any more. There will be more blood in the future."