Several thousand law enforcement agencies are creating the foundation of a domestic intelligence system through computer networks that analyze vast amounts of police information. As federal authorities struggled to meet information-sharing mandates after the Sept. 11, 2001 terrorist attacks, police agencies from Alaska and California to the Washington region poured millions of ... records into shared digital repositories called data warehouses, giving investigators and analysts new power to discern links among people, patterns of behavior and other hidden clues. Those network efforts will begin expanding further this month, as some local and state agencies connect to a fledgling Justice Department system called the National Data Exchange, or N-DEx. The expanding police systems illustrate the prominent roles that private companies play in homeland security and counterterrorism efforts. They also underscore how the use of new data -- and data surveillance -- is evolving faster than the public's understanding or the laws intended to check government power and protect civil liberties. Three decades ago, Congress imposed limits on domestic intelligence activity after revelations that the FBI, Army, local police and others had misused their authority for years to build troves of personal dossiers and monitor political activists and other law-abiding Americans. Since those reforms, police and federal authorities have observed a wall between law enforcement information-gathering, relating to crimes and prosecutions, and more open-ended intelligence that relates to national security and [politics]. That wall is fast eroding following the passage of laws expanding surveillance authorities, the push for information-sharing networks, and the expectation that local and state police will play larger roles.