Answer:
"Just one more thing..."
Columbo would often put his quarry on their back foot by coming across as harmlessly bumbling. Getting ready to leave, having given them little to worry about with one or two easy questions, he would give them just enough time to shrug him off and enjoy a moment of relief that their dastardly crime had avoided detection by this scruffy detective... then turn and say "just one more thing..." with a raised finger and an expression of confusion.
At this point, the viewer knew that Columbo was already onto the suspect. From here on, the killer would be forced to perform all sorts of contortions with their version of the truth, constantly compelled to provide explanations for holes in the case, or give their increasingly weak version of events until they were finally hung by their own convolutions.