Sometimes, we discover new techniques by accident. Such was the case when my coworker, Candice, received a vehicle to process for latent prints late one Friday afternoon.
The Arizona DPS Crime Lab in Tucson used to be in the old regional headquarters building that was never designed to house a laboratory. We did not have a special bay for processing vehicles, but would have them towed into the huge open bay the mechanics used. At a couple thousand feet of floor space, there was room in one corner for us to process vehicles. If we had to keep them there any length of time, we would encircle them in yellow crime scene tape.
When a car came in late one Friday, my coworker Candy began to dust the exterior with black non-magnetic powder using a fiberglass brush. She had completed the driver’s side, rear fenders forward to the front bumper, and had started on the hood when she quit for the day without developing a single latent print.
Candy returned on Monday morning and picked up where she had left off. The passenger side was covered with latents. Candy went back and re-powdered the driver’s side and developed just as many fingerprints there, too, prints that had not responded to powder the previous Friday.
We met about that and tried to figure out where all the latents had come from. Had highway patrolmen trapsing through the bay patted their hands and fingers all over the car? Not likely. They knew better than to handle evidence.
As most people know, Tucson is a desert town. The temperature is usually above 100° F in the summer and the humidity is in the single digits. As the locals like to say, “It’s a dry heat.” It was common that once a car had sat in the summer sun for any length of time, it would be impossible to develop latent fingerprints on the exterior. Even the sebaceous component of fingerprint residue would bake off in a matter of minutes in the intense summer sun.
But a technique we used on small items in the lab was “huffing,” or slowly exhaling hot breath on a surface to re-humidify a print while using magnetic powder. Finally it dawned on us that Candy had done something similar with the car she was processing. Inadvertently, it had been parked directly beneath the blower of a rooftop evaporative cooler. The car had cool, damp air blowing on it over the weekend, which had revived the latent prints so that the regular non-magnetic black powder worked very well after a few days of re-humidifying the prints.
Subsequently, when a car was brought in, we would have it dropped off under that evaporative cooler and let it sit at least overnight before we tried to process it. When the new lab was built, it had a special vehicle processing bay. I left shortly after we had moved into the new lab and had very limited experience with vehicles there, but thanks to Candy’s serendipitous experience in the old vehicle bay, we learned the trick of re-humidifying the exterior of a vehicle before writing it off as having no latent prints.