SAN FRANCISCO, May 9 - 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...