The bureau also said it was continuing to work with Swedish police on the case, as well as British authorities and law enforcement agents of several other European countries, to determine if there were accomplices in the 2004 breach.
Neither Cisco nor the FBI would comment on whether the stolen information had been used to launch attacks via the Internet, but a report in The New York Times claimed the codes had been used in ''broad and long-lasting'' attacks on high-security computer systems, including those of the U.S. military and NASA.
However, a spokesman for the FBI did say that the year-long investigation had effectively put a stop to any illegal activity surrounding the case.
As previously reported in April 2004 by internetnews.com, shortly after the breach, a Russian security Web site reported that hackers broke into the switching and routing giant's network and stole 800MB of source code for IOS 12.3 and 12.3t. Samples of the code were reportedly posted on an underground IRC channel as proof of the breach.
The Times report claims the attacks were detected by workers at several research labs who saw that software on Cisco computer routers had been compromised.