The hunt for who leaked Windows source code is centring on a small Silicon Valley firm called Mainsoft.
Mainsoft is helping with the investigation
Analysis of the code circulating online suggests it was being used by Mainsoft to help it create its own programs.
Microsoft has confirmed that the source code has been leaked but said it was not due to a breach of its security.
The FBI has also confirmed that an investigative task force had been set up to look into the how the source code got on the internet.
Mainsoft has had access to the Windows source code since 1994 and until 2001 was one of only a very select band of firms that Microsoft trusted with the raw computer code for its operating systems.
Analysis suggests the source code was used to help Mainsoft create its MainWin program that many firms use to make Unix versions of Windows software.
In files found with the chunk of source code is information that points to Mainsoft engineers.
Attention is focussing on a file called a "core dump" that is created during a crash and helps programmers work out what went wrong.
Mainsoft said it would cooperate fully with the investigation into the leak.
"Mainsoft takes Microsoft's and all our customers' security matters seriously, and we recognize the gravity of the situation," said Mike Gullard, chairman of Mainsoft.
Lots of organisations have seen Windows source code
Experts say that a mention of Mainsoft in the code does not mean the company is the source of the leak.
The chunk of code circulating is 205MB in size and is believed to be about 15% of the raw code for Windows 2000 and Windows NT4.
If Mainsoft does not turn out to be the source of the leak, tracking down just who did leak it might be difficult.
This is because Microsoft has recently begun sharing Windows source code with an ever increasing pool of organisations.
Researchers as well as key customers and software resellers can get a look inside Windows under the Shared Source Initiative University started in 2001.
Although not all the code is shared with these organisations it is thought that up to 90% of the source code for Windows has been seen by people outside Microsoft at some point.
Under this initiative, thousands of organisations and probably tens of thousands of people have viewed the raw code for Windows' various incarnations.
In October 2003 Microsoft extended the source code sharing to include its 1200 Most Valued Professionals - key computer technologists who specialise in Windows.
When news of the leak first broke, some security experts expressed surprise that the code had not been leaked before given that so many people have seen and tinkered with it.
However, many of those that get access do so via a variety of security mechanisms. Some have to use a smart card to access the site and others are only allowed to view the code in Microsoft buildings using computers controlled by the software giant.