Crimean bridge: Who - or what - caused the explosion?

What 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.