The FBI is helping Cisco to examine further evidence of how its source code was distributed.
More samples of the operating system code, IOS, are said to be circulating the web after a hacker took the code from a Sun Microsystems server at Cisco.
The hacker provided a temporary link to the code on a server at the University of Utrecht, according to Alexander Antipov, a security expert at Moscow security firm Positive Technologies.
Four days ago, a Russian website reported the code was stolen and connected with a man named 'Franz'.
The leak is similar to Microsoft's loss of its source code, which happened in February. The security of Cisco's source code, like Microsoft's, largely relies on the it being kept secret.
Cisco was unable to comment on the matter.