Police forces across the United States are stockpiling massive databases with personal information from millions of Americans who simply crossed paths with officers. A person can end up in one of these databases by doing nothing more than sitting on a public park bench or chatting with an officer on the street. Once there, these records can linger forever and be used by police agencies to track movements, habits, acquaintances and associations - even a person's marital and job status. What began as a method for linking suspicious behavior to crime had morphed into a practice that threatens to turn local police departments into miniature versions of the NSA. In the process, critics contend, police risk trampling constitutional rights, tarnishing innocent people and further eroding public trust.