The areas in this region are unbelievably huge, the country is so big that special forces could easily sneak past.What if it wasn't UKR at all. after all they would need to drive a truck with tons of explosives on board for 1000's of Km through Russia. What if it was an internal Russian faction, esp as there were all sorts of high explosive cigarette fires deep inside Russia(e.g all the fires inside Russia.. that Oligraghs daughter in a car bomb) the TU22's that got got last night deep inside Russa ) . So.. said faction could either be anti-putin or pro war, wanting to use this as an excuse to escalate even more(or both). esp as Russia was very quick to name the truck driver.. How would they know if the truck was...well nothing
Thats not taking into account that there will be a lot of special forces already in the area that do these jobs.
I believe there are special forces also inside Russia sabotaging and burning all these buildings, car bombings etc. People are falling out a lot of windows, it could be Putin or it could be special forces picking of vulnerable targets that don't get the same levels of protection Putin does.
Someone's doing this, it's not Russia as that bridge is a necessity for them.