How do crime scene investigators know that they've gathered all of the evidence,such as tiny amounts of blood?

how do they scan the scene for evidence which are very tiny and sometimes can't be seen with the naked eye without a microscope? a black light can see visible amounts of saliva,etc but won't it overlook a droplet of blood on a carpet or wooden floor or on a object? do skin cells contain DNA? do they use a lint roller or mini portable vacuum device to pick up evidence on the carpet or what devices to gather them without destroying the original form? I'd like to hear from people who are familiar with this or experienced it first hand or know people, who are in the industry to give information in your own words.thanks.

How do crime scene investigators know that they've gathered all of the evidence,such as tiny amounts of blood?
the scene is done in a number of patterned sweeps.


from left to right top to bottom.


then reverse it and go back over the scene from bottom to top right to left.


a circle pattern is done by some.


a x pattern by others.


a grid pattern by another.


sometimes to just see it from a different view.





there are powders, sprays etc that will enhance an ultraviolet lighting.





without destroying the original form. cannot be done. one you remove an object it is no longer a part of that object. this is why a patterned search is done and evidence gathered and logged.


a crime scene sketch is a picture of the scene drawn to scale. pictures enhance the scene as here we found a ---,mark it as exhibit ???, log it on the log sheet exhibit ???, sketch it in the diagram, photograph it, log photo number off camera as photo number-- relates to exhibit number--"as a---"





I have had on a homicide case as many as 125 photos and pinpoints where I felt as the investigator evidence was found and might be use later as the case developed.





never sweep anything it destroys the area you were working in. exactly where did this come from? diagram, photo, exhibit, measurement from 4 corners to the exact 1/16 of an inch.





you can never gather enough evidence. and you always feel you might have missed something.
Reply:They have a chemical that under UV light makes sperm and blood glow in the dark. Plus a boat load of trainnig.
Reply:There are special lamps that show the blood like a florescence glow which stands out brightly. Dogs also can be used. Bloodhounds are the most known for their great sence of smell. Also some sprays have been developed to make a great display when in contact with blood.





Best Wishes,





Officer H.
Reply:They are almost certain of this because blood has DNA in it, and you can find a person by just knowing their DNA.
Reply:You can have, and they do have, all kinds of special equipment but it all comes down to training and more importantly experience.


There is equipment like coherent light systems, UV light systems. There are sprays but despite what you see on TV Luminol only works to show were blood is for a small amount of time and you need good light control to view it, it also destroys much of the evidence that can be gleaned from the actual blood.


Mini and full sized vacuums are used. Again, the chain of evidence must be established and maintained so the machine is perfectly clean and the filter is special.


It is an on going process as to what can be used. Lint rollers have been used and so has adding machine tape (to get latent fingerprints from human skin).
Reply:They don't need it all, only enough to contribute to a ase that would be found to be "beyond reasonable doubt".





And of course the defense has the right and even duty to suggest that the case has not been made.

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