I’ve always wondered about these police procedures:

At a crime scene, there are scores of people around—police officers, EMTs, detectives, curious onlookers, victims, etc. Dogs are brought in to see if they can pick up the scent of the perpetrator(s). How do they know which of the many scents to follow? At times they use a sample of the victim’s clothing, but that isn’t always the case.

When searching for finger prints on some objects, like a public telephone or a rented car, there could be thousands of prints already on the item. How are the desired prints located?

Often the police are dissatisfied with the quality of the finger prints obtained. If everyone’s finger prints are unique I assume that applies to the entire body of skin. If the finger prints are smeared or otherwise not clear, why don’t they use a toe print or a print from another body part?

Does anyone have the answers?

I can only comment on the prints - you can't get prints from most places from the body. The fingers leave prints as they have skin ridges; skin on other parts of the body does not. I guess they use finger prints more than toe prints, well, because what are the odds of finding toe prints as opposed to finger prints? :P I do know that sometimes footprints are used, but more based on the weight bearing to tell if the person was walking or running for example, rather than matching the unique prints to a specific person.

When looking at finger prints they look for ridges, loops, arches and other patterns. Those ridges and loops are not found on other parts of the body (except for toes). They will look for toe prints or may find them, but how often would someone commit a crime barefoot? Generally if someone leaves toe prints around they will leave finger prints.

As for not using a print from another part of the body, they do do that. Forensics can use hairs, saliva, sweat, blood, etc and get DNA from it. That's a form of a print, in my opinion.

There aren't as many usable prints as you might think on objects such as public pay phones and rental cars. But the techs would take any print they could find that wasn't completely useless, run them all throw the database to look for matches. The prints probably wouldn't be usable in prosecution without corroborating evidence anyway, so all they could hope to get is a lead.

Here's a little reading for you about the subject: http://www.essortment.com/all/fingerprintssec_rwkx.htm

As for the police dogs, they pick up the scent of humans. If you have a piece of clothing from the suspect, the dog would know the smell of the dead skin cells from the person that used it and that's what they would track. A little info about that is at the bottom of this page http://hubpages.com/hub/K-9-Police-Dogs under "How Police Dogs do their work".

A K-9 unit is generally one of the first units on site, so there wouldn't be as many people around as you would generally assume. The suspect would have had to come from somewhere or be last seen somewhere, so that would give the officer a place to start with the dog. They go from there. The dog doesn't lead the handler to the suspect in 15 seconds like they always show on TV, it can take some time. They also usually have a specific area they know the suspect is in so it can be easier for the dog to seek out the suspect. Also, generally, the suspect will be sweating, breathing hard and scared. All things that the dog can use to help pick up a stronger scent or hear him or her.

Aren't lip prints on glasses and bottle tops sometimes used, especially if they are lipstick type prints? I would guess the patterns of grooves on lips are unique from individual to individual.

Thank you all for your interesting and relevent information.

They are more likely to get DNA from the saliva left behind on the glass.

I guess that dogs (and their handlers) are just that good, the mental picture is not as confusing for them as it is for us. Conversely, dogs may be bewildered by the fact that we can identify people by just looking at their faces, but without sniffing.


With a LOT of patience; noone said it's an easy job :P


They do. Burglars put their ear to a window to listen if someone is inside before they break in. A couple of months ago, someone here was found guilty because he was identified by the earprints he left on his victims' windows.

Somewhat off-topic, but not completely. There was an episode of Dragnet in 1969 where they trained a dog to pick up a scent. Dogs can pick up a scent rather quickly. The investigator probably has something with the perpetrator's scent, and gets the dog zeroed in on that sole scent.

This post is fantastic! I'm a CSI freak anyway!Thanx for posting!

If dogs are used they will follow any scent they pick up. Trained dogs, and they don't take too much training to follow their instincts, are used to track a particular scent. They need to know which one out of the hundreds of trails to follow. Sometimes they are used just to try and find out if anyone has been around the area. If a dog is used and the suspect is still close they will either try and hide or run. If the dog shows interest in an area that is unusual the handler will investigate. If the suspect runs the dog is sent in pursuit and 'locks in' to that person. The chase instinct is very strong in certain breeds and they are not easily distracted. I've seen it many times walking my own dogs (both German Shepherd).