China could be a culprit, as well could be a Middle-Eastern country. There is no one that should be ruled out unless proven to not be corrupt in this case. Our so-called ally, China, likes to strong-arm others politically. It could be North Korea as well. The truth is it could be over a handful of different countries.
The answer is always Russia. If it is not Russia, it is the Ukraine or some other Eastern European country with cold weather and funny hats. The Cold War has never really ended. We say they embraced democracy but no government is truly democratic especially our own, and governments don't change that much.
The presidential campaign is a particularly dirty one this year. If Putin says that Russia is not trying to dirty the DNC he may or may not be telling the truth. Any number of entities, from Russia, to a Trump PAC, to a terrorist organization, to individuals simply against Clinton's candidacy could be pulling dirty tricks on the DNC.
There is evidence that someone in Russia hacked the DNC. This may not necessarily reflect the Russian government. It could very well be an individual who did it of his or her own accord with no input or support from the government. It could also be possible we do not actually know the truth. We just wanted someone to blame.