Photos released by the Ukrainian government as "proof" of Russian soldiers on the ground in Donetsk leave many questions to be answered.
With thousands of Russian soldiers massed on Ukraine's frontiers, any evidence that they are actually over the border, as was the case in Crimea in March, is being taken very seriously.
What do the photos show?
Heavily armed gunmen in combat uniform, many of them masked.
The low-quality, annotated images were handed to the Organization for Security and Co-operation in Europe on 16 April, as evidence that Russian "sabotage-reconnaissance groups" had been working with separatists in the Donetsk towns of Kramatorsk and Sloviansk.
According to a New York Times article, the photos and their descriptions were "endorsed by the Obama administration". Distributing the images, the US state department said they confirmed the "connection between Russia and some of the armed militants in eastern Ukraine".
However, the BBC has been unable to verify the pictures and there was no immediate response from the Russian government.
How damning are they?
The Ukrainian press release says the photos show the same heavily bearded gunman taking part in militant operations in Kramatorsk and Sloviansk this year, and in an operation in Georgia in 2008, when Russia fought a brief war with that former Soviet republic.
This would be damning evidence indeed but in the 2014 photos, the man's greying beard appears to be black while in Georgia six years ago, the slimmer-looking man shown has a reddish beard.
The Ukrainian government highlights a Russian special forces badge on the sleeve of the gunman in Georgia but such badges can be bought on the internet for less than $5 (£2.90).
Another set of photos purports to show the same masked gunman in both Crimea earlier and in the Donetsk region this month. However, while a similar combat uniform is worn in both photos, the masks are different, as is the way the pistol is worn on his belt.
Apart from the photo said to have been taken in Georgia, all of the images seem to be recent and there is nothing to suggest any of them were taken outside Ukraine.
Do they prove anything?
What comes across from the photos is that at least one unit of heavily armed, well-equipped, pro-Russian paramilitaries has been operating in the Donetsk region.
But it cannot be said for sure that they are actual Russian special forces, as the Ukrainians argue. At the same time, the idea that they might be a local militia from Donetsk is belied by their apparent military professionalism.
Russian President Vladimir Putin said last week there were "no Russian units, special services or instructors in the east of Ukraine" but such denials of military involvement ring hollow for many after Russia's covert actions in Crimea, especially after he subsequently admitted troops had operated there.
That said, theoretically, the paramilitaries in the photos could be ex-servicemen from Russia or elsewhere. Military veterans of the Soviet war in Afghanistan have been active on both the nationalist side in Ukraine and in patriotic groups in Russia.
Will more evidence emerge?
There are several clear photos of three of the gunmen's faces so their identity may be established eventually. However, the fact that they all have beards may complicate matters.
An article about covert warfare on the US website Defense One argues that iris scanning is a good way to identify masked men but admits the limitations to this approach in Donetsk. "Ukraine isn't Facebook," it notes.
Meanwhile pro-Ukrainian bloggers share photos of militants in the east, both masked and unmasked, in the hope that someone may be able to recognise them.