Russian hacking was exposed in a detailed indictment of 12 Russians by the US Department of Justice (DoJ). While reading through it I was intrigued at how the indictment laid out methods that the Russians used in clear and simple English.
Reading through the indictment, it gave all the classic indications of an advanced persistent threat (APT), the indictment never said so. Instead it indicated the attacks came from Units 26165 and 74455, which are part of a Russian military agency called the Main Intelligence Directorate of the General Staff. This is commonly abbreviated as GRU.
Read more about it here.