The incident seemed alarming enough: a breach of a Cisco Systems network in which an intruder seized programming instructions for many of the computers that control the flow of the Internet.

Now federal officials and computer security investigators have acknowledged that the Cisco break-in last year was only part of a more extensive operation - involving a single intruder or a small band, apparently based in Europe - in which thousands of computer systems were similarly penetrated.

Investigators in the United States and Europe say they have spent almost a year pursuing the case involving attacks on computer systems serving the American military, NASA and research laboratories.

The break-ins exploited security holes on those systems that the authorities say have now been plugged, and beyond the Cisco theft, it is not clear how much data was taken or destroyed. Still, the case illustrates the ease with which Internet-connected computers - even those of sophisticated corporate and government networks - can be penetrated, and also the difficulty in tracing those responsible.

Government investigators and other computer experts sometimes watched helplessly while monitoring the activity, unable to secure some systems as quickly as others were found compromised.

The case remains under investigation. But attention is focused on a 16-year-old in Uppsala, Sweden, who was charged in March with breaking into university computers in his hometown. Investigators in the American break-ins ultimately traced the intrusions back to the Uppsala university network.

The link for this article located at The New York Times is no longer available.