I'd venture that the mere fact that they have done any attack once to any coin is enough to cast suspicion on them as a suspect, as it is a display that they are not 'above' doing such a thing, while most other people would never consider doing such a thing, it is never really acceptable to attack any other coin no matter what the circumstances...
I (and many others) could do immense damage right now if I wanted to, to any number of altcoins that are on old code bases or are PoW without check pointing, but we won't because we have ethics.


However that aside, unless someone has some actual proof (in which case please do come forward with it) it is all just speculation, and I don't think speculation can cause anything other than harm, so I say let us move on and focus on what we have always done building something awesome, let us not associate with those who only want to tear things down and destroy.
We can sit and make theories about who did this, or we can move on and do something constructive, I choose the second option and I hope that everyone else will also.