By Paul AdamsBBC News Kyiv, Ukraine
Watch: The moment explosion hits Crimea bridgeWhat do we know about what caused Saturday's dramatic explosion on the Kerch Bridge?
There are plenty of theories, not all of them very credible.
Russia was quick to suggest this was a truck (lorry) bomb, but didn't say who orchestrated it.
Russian President Vladimir Putin accused Ukraine of attacking the bridge in an "act of terrorism".
Security camera footage released on social media showed a truck - allegedly from the Russian city of Krasnodar, an hour's drive from the crossing - moving west across the bridge at the time of the explosion.
Russian officials named a 25-year old Krasnodar man, Samir Yusubov, as the owner of the truck, and said an older relative, Makhir Yusubov, was the driver.
But close examination of the footage seems to show that the truck had nothing to do with the explosion.
The footage shows a huge fireball erupting just behind - and to one side - of the truck as it begins to climb an elevated section of the bridge.
The speed with which the truck bomb theory started to spread in Russian circles was suspicious. It suggested the Kremlin preferred an act of terrorism to a more alarming possibility: that this was an audacious act of sabotage carried out by Ukraine.
"I've seen plenty of large vehicle-borne IEDs [improvised explosive devices] in my time," a former British army explosives expert told me. "This does not look like one."
A more plausible explanation, he said, is a massive explosion below the bridge - probably delivered using some kind of clandestine maritime drone.
"Bridges are generally designed to resist downwards loads on the deck and a certain amount of side loading from the wind," he said. "They are not generally engineered to resist upward loads. I think this fact was exploited in the Ukrainian attack."
Some observers have noted that in one of the other security camera videos, something that looks like the bow wave of a small boat appears next to one of the bridge supports, a split second before the explosion.
What kind of vessel could it be?
Watch: Is this what caused the Crimean bridge explosion?On 21 September, images circulated on Russian social media channels showing a mysterious unmanned boat that washed ashore near Russia's naval base in the Crimean city of Sevastopol.
It resembled a large black covered kayak, complete with bow-mounted sensors and a white, periscope-like device on top
According to local reports, the vessel was towed out to sea and blown up.
"A part of an unmanned vehicle was discovered," the governor of Russian-controlled Sevastopol is quoted as saying.
"After the survey was completed, this apparatus was destroyed at sea by an explosion. No-one was hurt."
This is not the first time reports have circulated suggesting that Ukraine has access to such clandestine equipment.
"There are well-founded reports which suggest that the Ukrainians have both surveillance and strike maritime remote controlled vehicles in service," the British explosives expert told me.
"This operational concept has been developed over years, not months."
If this is how Ukraine managed to attack the Kerch Bridge, hundreds of miles from Ukrainian-controlled territory, then it's one of Kyiv's most ambitious operations so far.
But apart from a few whispers in the capital, no-one is confirming the theory.
The $3.7bn (£2.7bn) bridge links southern Russia with the territory it annexed in 2014 (2018 report)In fact, in a statement last night, the head of President Zelensky's office, Mykhailo Podolyak, seemed to endorse Moscow's truck bomb theory.
"The answers should be sought in Russia," he said in a statement.
The explosion, he said, was the result of infighting between different parts of Russia's security establishment.
"This is a concrete manifestation of the conflict between the FSB [Russia's internal intelligence service] / PMC [private military contractors, like the Wagner Group] on the one hand, and the Ministry of Defence / general staff of the Russian Federation on the other hand," he said.
Did Mr Podolyak know something everyone else didn't? Or was he, perhaps, trolling Moscow, playing on extremely raw nerves exposed by Russia's recent setbacks on the battlefield in Ukraine?
The truth is, we don't know.
Just like previous episodes - including the sinking of the flagship of Russia's Black Sea Fleet, the Moskva, and the mysterious attack in August which devastated a Russian airbase in Crimea - Kyiv is very happy to keep everyone guessing.
It's all part of a highly successful information campaign which Ukraine has waged, along with its military effort, ever since Russia's full-scale invasion in February.
For now, it seems to be working.
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