Two Suggestions
One way to help solve crimes like this would be to have unique serial numbers or series of symbols and characters stamped onto the head of all hammers. That way, the medical examiner's office can get these tool mark impressions from the victim's corpse, and the hammer can be traced back to the killer, and the crime is solved. After many such crimes are solved, killers will stop using hammers to kill, and we'll all be safer.
Or we'll have to start regulating the sales of files and sandpaper and grindstones...
Second point: Rather than identify the bad guys by tracing hammers, I propose to do it with fingerprints and DNA.
After a couple of the old women pull pistols from their purses and empty them into the bad guy, the cops can swap generous samples of his blood and body tissues from the floor and walls of the business. If the women are good shots, or just lucky, the bad guy's fingerprints will be available, found on the end of his bloody fingertips, out in the parking lot, still attached to his rapidly-cooling corpse. This will lead to his identification with a high degree of scientific certainty, and the police will be able to close the case without a trial.