On August 21, 2015, our Harris County District Attorney provided a Brady notice regarding DNA mixture cases. While the office had previously provided notice of the FBI STR population database errors, it was generally believed that the errors would not be significant. However, we learned that when the error combined with DNA mixture analysis, the effect can be quite significant.
The DA’s notice came with the attached analysis from the Texas Forensic Science Commission.
Here, the commission distinguished the FBI database error from the changing standards in CPI/CPE interpretations that apply to mixture cases. A mixture case is one in which more than one DNA profile is identified within a particular sample. This means the sample collected, say from a crime scene, contains a mixture of DNA from more than one person.
The commission points out that it is the FBI error together with the CPI standards that has led to more significant discrepancies in reporting.
A more recent memo (September 10, 2015) from the Texas Department of Public Safety Crime Lab explains that its lab has only this year updated its standards in mixture cases, resulting in all cases since 1999 (when they started STR analysis) until August 10, 2015 (when they changed their CPI protocol) being suspect.
Additionally, in a July 28, 2015 audit of the Houston Forensic Science Center (the former HPD Crime Lab), conservative practices have been criticized as perhaps failing to identify and thus provide probative exculpatory evidence. (see page 3) This audit report was not received until September 18, 2015 when the DA’s office forwarded the audit and an internal response to the audit.
As you can see, new information is coming out almost monthly. We will continue to provide information as we receive it.