The Chemistry section performs analysis on two different types of evidence submitted by law enforcement agencies: alleged controlled substances and primer gunshot residue. The section examines solid and liquid samples (plants, powders, oils, tablets, etc.) for the presence of controlled substances. To meet accepted forensic standards the positive identification of a controlled substance requires two positive results from two diverse analytical methods. This is done primarily through liquid chromatography and gas chromatography/mass spectrometry. Additionally, the section performs primer gunshot residue analysis using scanning electron microscopy. SEM gives an elemental image of particulates found in primer gunshot residue.

Evidence Receiving

 The Evidence Receiving Section’s function is an integral and essential part of the Bexar County Criminal Investigation Laboratory as it is responsible for receiving, storing, transferring, and releasing all evidence submitted by the investigating agencies as needed to complete analytical requests. The section also handles the associated data entry, record retention, and invoicing.

The section’s primary objective is to safeguard all evidence in the custody of the Bexar County Criminal Investigation Laboratory and to maintain the integrity of an accurate chain of custody for each item of evidence.


The Firearm & Toolmark Section examines and test fires firearms to determine their operability. Toolmarks present on fired bullets and cartridge cases are examined and microscopically compared to determine if they were fired in or from a particular firearm. Additional services performed by this Section include the restoration of obliterated serial numbers, estimation of firearm muzzle-to-target distance using gunshot residues, and examination and comparison of non-firearm tools, footwear, and tire track evidence.

Bullet ComparisonCartridge Case Comparison


The Serology/DNA section performs analysis on physical evidence from criminal and medical-legal investigations.  Services include, but are not limited to, chemical testing for the presence of biological fluids (such as blood, semen, and saliva), detection for the presence of male DNA (Y-Screening), and swabbing items of evidence for DNA deposited by the handler or wearer of an item.  Forensic DNA analysis is subsequently conducted on items of evidence to develop a genetic profile that may be compared to the known genetic profile from a person of interest.  The analyst may draw a conclusion to determine if the individual can be included or excluded as a potential contributor of the DNA observed on the item.  If an individual cannot be excluded as a potential contributor of the DNA, the rarity of the genetic profile will be provided to help determine the weight of the evidence.   

Additionally, the DNA analysts may perform Y-STR testing, a specialized form of DNA analysis targeting the Y-chromosome found in male individuals.  This technology is especially useful in sexual assault case samples where the amount of female DNA may overwhelm traditional DNA testing methods.

DNA profiles can be entered into a genetic database called the Combined DNA Index System (CODIS) and are routinely searched against the genetic profiles developed from crime scene samples, convicted offenders, and arrestees.  The searches may be conducted at the local, state, and national levels providing investigative leads to law enforcement agencies. 

Unless otherwise specified in a laboratory report, genetic profiles entered into CODIS are uploaded to the National DNA Index System (NDIS) and/or State DNA Index System (SDIS) and searched indefinitely. Should the genetic profile(s) match to another genetic profile in CODIS, the investigating agency will be contacted, as appropriate.