A United States Secret Service study concluded that schools were placing false hope in physical security, when they should be paying more attention to the pre-attack behaviors of students. Zero-tolerance policies and metal detectors "are unlikely to be helpful," the Secret Service researchers found. The researchers focused on questions concerning the reliance on SWAT teams when most attacks are over before police arrive, profiling of students who show warning signs in the absence of a definitive profile, expulsion of students for minor infractions when expulsion is the spark that push some to return to school with a gun, buying software not based on school shooting studies to evaluate threats although killers rarely make direct threats, and reliance on metal detectors and police officers in schools when shooters often make no effort to conceal their weapons.