On a summer evening in 2004, as the Supercomm tech conference in Chicago wound down, a middle-aged Chinese visitor began wending his way through the nearly abandoned booths, popping open million-dollar networking equipment to photograph the circuit boards inside, according to people who were there.

A security guard stopped him and confiscated memory sticks with the photos, a notebook with diagrams and data belonging to AT&T Corp., and a list of six companies including Fujitsu Network Communications Inc. and Nortel Networks...