Donna Alessi did not want to talk to the detective who he showed up at her door, so the detective waited for her to begin driving to work and used his emergency lights to pull her over – not because she had committed a traffic offense, and not because he suspected she was about to commit a crime, but because in this convenient manner he could override her will. The detective proceeded to question her. His purpose, he testified, was to talk to her as a witness in an ongoing investigation. His purpose was constitutionally inadequate to justify detaining her. But even if the detective had reasonable suspicion that Ms. Alessi had committed a burglary—as she was eventually charged—he had no authority to make a warrantless stop, with less than probable cause, on the basis of that past, completed crime.