"We see signs of Russian nationalism at work here, but no Russian government connection," Nazario told Heise Security. "None of the sources we have analyzed from around the world show a clear line from Moscow to Tallinn; instead, it's from everywhere around the world to Estonia."Link
The article also notes "there was no apparent attempt to target national critical infrastructure other than internet resources."
Update: Elver sez, "As all attempts to link the Russian government to the attacks were failing, the Russians themselves came and confessed! An official operative of Nashi, the youth organisation with close ties to Putin, gave an interview to the newspaper "Vedomosti" saying that he and his fellow Nashists were behind the cyberwar attacks. I only have an Estonian source for this, however.
"So while there's no evidence of a clear and simple connection between the Russian government and the attacks, operatives of Nashi have confessed to the attacks and the Nashi organization has very close ties to Putin."