1/ I'm not going to try to suggest definitively what caused the explosion on the Crimea/Kerch Bridge, but I thought I'd suggest a useful way of thinking about it. A short 🧵 on the subject.
2/ Lots of theories have been suggested – a long-range missile strike, a truck bomb (the preferred Russian explanation), a boat bomb under the bridge, some kind of sabotage. It was clearly a very big explosion, in any case.
3/ The UK has unfortunately seen a lot of explosions of this kind due to the Northern Ireland conflict. This video is a police helicopter's view of the IRA bombing of Manchester city centre on 15 June 1996. It provides an interesting point of comparison.
4/ The explosion caused the equivalent of $1.5 billion in property damage and made a crater 15m (45 ft) wide, but didn't kill anyone. The bomb weighed 1,500–1,600 kg (3,300–3,500 lb) and was made from a mixture of the plastic explosive Semtex and ammonium nitrate fertiliser.
5/ The vehicle carrying the 1996 Manchester bomb was a 7.5 ton Ford Cargo van. The truck implicated by the Russians appears to be an International ProStar, weighing between 14.5-27.2 tons (32,000-60,000 lb). If a bomb was aboard, it could have been far bigger than in Manchester. 
6/ The suspect truck was given a brief and apparently only cursory inspection by bridge security guards at 05:52 before it was allowed to pass. It reportedly exploded around 06:05, shortly before dawn.
7/ I wouldn't be surprised if the truck's arrival took place just before a shift change by the bridge security guards. At that time the night shift were likely hungry, tired, and cold, and would have been focusing mainly on going home to get warm and rested.
8/ In considering which of the possibilities I mentioned above are the more likely, it's worth bearing the principle of Occam's Razor in mind.
9/ This says (per Merriam-Webster) "that the simplest of competing theories be preferred to the more complex or that explanations of unknown phenomena be sought first in terms of known quantities."
So how does this inform views of the competing possibilities? It's quite useful.
So how does this inform views of the competing possibilities? It's quite useful.
10/ Long-range missile: an unknown quantity. There's no clear indication that Ukraine has anything like this, whether the US ATACMS or a home-grown alternative.
Explosive boat: also an unknown quantity, same reasoning.
Explosive boat: also an unknown quantity, same reasoning.
11/ Sabotage: Ukraine has certainly been able to strike behind enemy lines, as various places in Crimea have discovered. So this is a known(ish) quantity.
12/ Vehicle bomb: As with sabotage, car bombs have certainly been used, as this assassination of a collaborator in occupied Starobilsk in August shows. A truck bomb is just scaling up. Another known quantity.
13/ Let's eliminate the two unknown quantities, leaving us with sabotage and a truck bomb. Comparing the Manchester and Crimea Bridge explosions, it's clear that the bridge explosion was much bigger. More explosives or more powerful explosives, but likely several tons' worth.
14/ Video footage from the rail bridge shows that there were cameras mounted underneath it to monitor possible intrusions (no boats were supposed to be under there, they go under the main span).
15/ Common sense suggests that any attempt to plant several tons of explosives on the bridge would have been easily spotted. Which likely rules out the possibilty of sabotage.
16/ So, thanks to Occam's Razor, it's fairly safe to say that the Russian theory of a truck bomb is the most probable. That's not to say it's correct – but it's the least complicated explanation and is most within the area of known quantities. /end
Some views linked below from a Finnish explosive ordnance disposal expert:
https://twitter.com/ChrisO_wiki/status/1579480666282287104
Further commentary and observations on Tyry's interview here.
https://twitter.com/ChrisO_wiki/status/1579511852815781888
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