Processing a Damp Pistol and Plastic Bag for Prints and DNA
A 54-year-old homeowner is found dead in the kitchen with a plastic grocery bag near his head. A loaded 9 mm pistol lies on the carpet 1.5 m away, a drinking glass is on the counter, and a partially eaten meal is on the table. The outside door handle is smudged, and the thermostat reads 22 degrees C with 65 percent relative humidity. The pistol's grip and the bag are both candidates for latent prints and DNA. The bag is crumpled and slightly damp; the pistol has visible moisture on the slide. A cyanoacrylate chamber is available, but the laboratory is 90 minutes away. The detective asks you to process the bag and pistol for prints before they are sent to the lab.
1. What processing sequence would you use for the pistol and the bag, and why?
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Sequence from least destructive to most destructive. First document and photograph both items in place. For the pistol, consider DNA swabbing of the grip and trigger before any print powder, because powder and cyanoacrylate can interfere with DNA. If the pistol is damp, air-dry it and package it in a breathable container. For the bag, cyanoacrylate fuming is often better than powder on plastic because it develops latent prints without the mechanical damage of powder; but the bag is crumpled and damp, so dry it first. Do not process the bag with powder before fuming. The 90-minute trip to the lab is acceptable if the items are packaged and secured; processing at the scene increases contamination risk.
2. How do you decide between latent print processing and DNA sampling on the same surface?
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Latent print processing and DNA sampling compete for the same surface. Decide based on the item and the question: the grip of the pistol may hold touch DNA, while the smooth slide may hold prints. Swab DNA first in areas least likely to hold ridge detail, then fume or powder. For the bag, prioritize fuming because the plastic surface is favorable for cyanoacrylate and the bag is a likely source of prints. Use a clean chamber and a control. Document the order. If you powder first, you may still get DNA, but the powder can clog swabs and reduce yield. The limitation is that you cannot know which surface holds the best evidence without testing, so preserve both with sequential sampling.
3. How should the pistol and bag be documented and packaged for transport to the laboratory?
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Photograph the items before packaging, with scales and orientation. Package the pistol in a rigid, breathable container or a firearm box, secured with zip ties, and label the magazine separately if removed. Package the bag in a paper bag after drying; do not fold it in a way that concentrates trace evidence. Use separate containers for each item and change gloves. Record the chain of custody, including the moisture condition. Do not transport the pistol loaded in a way that risks discharge; render it safe according to policy and document any manipulation. The meal and glass can be collected later, but the glass is also a DNA and print surface and should be photographed before handling.