LA Forensics: The Signature Murders
Larry Blanton was a supervising criminalist for the LAPD at the time of the Garcia murder. He explained how they used the PCR (polymerase chain reaction) method for extracting DNA from the blood samples. "The DNA is relatively stable," he said, "as long as there's no excessive heat or moisture." DNA testing confirms the source, so most labs no longer do ABO type testing or testing for species.
Nowadays, the lab uses the Kelex process, or the capillary electrophoresis. Once a sample is in the test tube, they apply a Kelex resin, which binds with the impurities in the blood to get them out of the way. The resulting DNA is cleaner.
Then the DNA gets extracted from the white blood cells. When the sample is heated in a water bath, the lab gets a printout of how many nanograms of DNA they have. Next, for about two hours, a thermal cycler multiplies the sample, making billions of copies by running cycles of higher and lower temperature changes. The samples are sucked into a needle to travel through a thin capillary, so they can be detected with a special camera. A laser beam hits the specimens and the software analyzes the results.
Or, the DNA is placed in a solution over plastic strips, which have DNA probes on them. DNA from the sample adheres to the probes and when they change color, they indicate the sample's type.
In the Luis Garcia case, SID did two rounds of analysis. The first round involved nine separate blood stains. Seven matched the victim, one appeared to be a mix that included the victim, and one was foreign to the victim. "That was the one we were very interested in," says Blanton. "That represents blood from a person that has fled the scene."
The criminalists were able to convey to the detectives that if they developed a suspect, SID had a blood sample that could be compared.
The second round of testing involved bloodstains collected from the bedroom. These, too, were foreign to the deceased. Among the probative items were the store coupon, some tissues, a jewelry box in the bedroom, and a white shirt in a dresser drawer.
It was a good thing they had the blood evidence, because all of the fingerprints collected that were usable had been traced back to the victim or to family members.
However, without a suspect, just having DNA meant little. Soon, they ran out of leads.