Russian state TV channels have broadcast footage of men confessing to a plot to carry out terror attacks on the peninsula that was annexed by Russia in 2014.
The suspected saboteurs tell interrogators they were acting on orders from Kiev. Ukraine denies any involvement and calls it a provocation.
Local residents near the scene do report hearing shots fired last Saturday night, when the FSB security service says it intercepted the first group of men.
But the FSB statement describes a second attempted incursion of Crimea accompanied by "massive fire from the neighbouring state and armoured vehicles of the Ukrainian armed forces".
No video footage or independent confirmation of that incident has yet emerged.
What actually took place therefore remains a mystery. But it is what happens next that matters most, as the incident raises tensions between Moscow and Kiev to dangerous heights.
So is Moscow planning to retaliate militarily against what it is describing as an act of terror masterminded by Kiev?
Vladimir Putin has already pledged that he will "not let such things pass".
In that context, the subsequent deployment of S400 air defence missiles to Crimea could well seem menacing. Russia's Defence Ministry has also announced military exercises on the peninsula next week.
It all underlines Russia's intent to defend the territory it annexed from Ukraine two years ago in a move condemned as illegal by Kiev and the West.
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