Sinfully Rich by Hulbert Footner — Read

Once the police got their hands on the real criminals, their task was much easier. Many additional bits of evidence were forthcoming. The taxicab driver was found who had picked up MacKelcan and Lottie Bedford as they ran from Mike's apartment. This man had driven them to Grand Central where, no doubt, they had taken another cab to MacKelcan's office. Late that night, when the streets were cleaned, a sweeper in Lexington Avenue picked up the two pairs of keys where they had been dropped from the taxicab window, and turned them over to the police.

Witnesses were found who were prepared to testify that on several occasions they had seen either MacKelcan or Lottie in the neighborhood of Mike's apartment. Lottie, upon being taken to Stamford, was identified by a post-office employee as a woman who had been seen to visit the letter box hired in the name of John Jones. A search through Dr. Patton's letter files turned up a letter signed by "John Jones" of Stamford. It was Dr. Patton's answer to this letter which had served as a foundation for the order for hypodermic syringes. John Jones' communication was written on a typewriter in MacKelcan's office. So it went. Finally, a locksmith was found who identified Lottie as the woman who had ordered him to make the two keys for Mike's place from samples which she brought with her.