"The hackers, who have not come forward to claim responsibility or otherwise been identified, likely made it a “worm,” or self spreading malware, by exploiting a piece of NSA code known as “Eternal Blue” that was released last month by a group known as the Shadow Brokers, researchers with several private cyber security firms said."

Not saying it isn't impossible to happen, but consider this... in one month "hackers" were able to analyze the "Eternal Blue" code, figure out how it works, modify it or create their own code then distribute it to 57,000 systems world-wide where it then became active yesterday.  My gut has me thinking that work on all of this started before "Eternal Blue" was released a month ago.