When processing crime scenes, you will encounter many different types of evidence. Often, the most useful evidence will be DNA evidence. In order to make the most of this evidence, you need to know where to look for it, how to collect it, and how to package it. If you put in the effort to do the job right, you will be rewarded with an even stronger case.

The most common places to find DNA are in blood, semen, hair, and saliva. Any time you find these substances at a scene, you’ll need to collect them for DNA. You can also find DNA in urine, skin cells, perspiration, teeth, bone, and internal organs, so you need to examine your scene carefully. For example, if a bullet goes clean through the suspect, the bullet itself can be tested for traces of DNA. DNA can also be left on a variety of objects that people have touched or worn. We’ve found DNA on everything from dirty laundry to eyeglasses, cigarettes, bottles, and drinking glasses, partial fingernails (both broken and clipped), masks, gloves, and bandanas. All of these items should be collected and tested. In addition, you should look for DNA in the same places where you would look for fingerprints: the steering wheel of a car, door knobs and handles, counters, and cabinets, etc. All of these surfaces can be swabbed and tested for the presence of DNA.

Remember that when you are working a case with DNA evidence, you also have to identify the victim’s DNA and the DNA of anyone else who had access to the evidence.What this boils down to is you need to be prepared for what happens when you go to court. If you have any evidence that doesn’t belong to the suspect, the defense lawyer will try to claim that the DNA must point to the real perpetrator. So you have to be able to identify the DNA on all of your evidence to protect your case against the suspect.

You also have to protect your case by protecting the integrity of the evidence. In situations where you are collecting DNA, the key thing is to guard against contamination. Testing has become so sophisticated that we can identify DNA in very small samples, but it is very easy for the sample to become contaminated. First of all, you have to wear gloves when you are collecting evidence, but that’s just the first step. If you pick up one piece of evidence and then pick up another piece of evidence you can transfer evidence from the first item to the second item. You can avoid this kind of cross-contamination if you remember to change your gloves before handling each piece of evidence. You can also transfer evidence if you pick up an item then touch your face or hair, etc., with your gloved hand and then touch the item, so be aware of what you are doing at all times.

Next, you can prevent contamination by wearing a mask, since you want to avoid coughing and sneezing around the evidence you are processing. Also avoid touching areas where you might find DNA. Don’t grab an item the way you—or the suspect—would normally grab the item. If DNA is present, you’re likely to find it in this spot, so protect it by not handling it. Also, when possible, use disposable tools, such as disposable tweezers, to handle evidence. If you don’t have disposable tools, clean the tools thoroughly between dealing with each piece of evidence to avoid cross-contamination.