Last month, the Stuxnet rootkit seemed to come out of nowhere, exploiting a previously unseen, zero-day shell vulnerability in most versions of the Windows operating system.

But Symantec has found that the malware is anything but new. To be precise, Stuxnet -- or at least earlier versions of it -- dates from at least June 2009.

On Monday, Microsoft patched the Windows Shell vulnerability targeted by Stuxnet. But questions remain surrounding Stuxnet, including who built it, and why.

Some answers, albeit limited, are now starting to emerge, including the fact that Stuxnet isn't new. "The threat has been under continued development as the authors added additional components, encryption, and exploits," said Symantec's Liam O Murchu, who has been studying samples of Stuxnet to learn more about the attack and documenting his findings on a Symantec blog.

"We were interested to discover if the different samples we have seen in the wild were different variants or just modifications to the wrapper with the same components embedded," he said. "Analyzing the different types of samples we have observed to date has shed some light on how long this threat has been under development and/or in use."

Surprisingly, based on his research -- admittedly, into but a subset of all the different versions of Stuxnet to be found in the wild -- he found four distinctly different types of the malware, with the oldest dating from over a year ago.

Another interesting finding is that the most recent sample of Stuxnet is smaller than the oldest sample, even though the newer version contains more functionality. "Generally, threats grow larger over time, so it is not unusual to see that the newer sample has more resources -- 14 as opposed to 11 -- but it is surprising to see that the newer samples are smaller than the older samples," said O Murchu.

The evolution of Stuxnet -- as well as its targeting of Siemens industrial automation control systems, rather than trying to build a bigger botnet -- suggests significant brains, if non-obvious motives, behind the malware.

"The amount of components and code used is very large," said O Murchu. "In addition to this, the authors' ability to adapt the threat to use an unpatched vulnerability -- to spread through removable drives -- shows that the creators of this threat have huge resources available to them and have the time needed to spend on such a big task."

In other words, "this is most certainly not a 'teenage hacker coding in his bedroom' type operation," he said.