https://en.wikipedia.org/wiki/Baltic_Pipe
Putin was still trying to energy blackmail Europe back then. It is hard to see the explosions as anything else but a threat that the Baltic Pipe could also be blown up -- and the Nord Stream pipes weren't very useful to Russia at that point so it wouldn't cost much to lose them.
Also, given the climate now, if there was even a shred of evidence or any hint that Putin did this, US media and intelligence officials would be blaring that from every rooftop and every talking head would be "Russia this", and "Russia that". I think the relative silence speaks very clearly.