Matheson said the crime lab examination consisted of conventional tests and not the comparatively new DNA tests, the results of which are expected to be introduced later in the trial. The more traits examined, he said, the narrower the search becomes as more and more people are excluded as the possible source of the blood. Matheson also testified about the blood found under Nicole Simpson's fingernails at the crime scene. The defense contends the blood does not match either of the victims or Simpson, but a fourth person who is the real killer. But the prosecution argues, and Matheson testified, that the blood under the nails does not match Nicole Simpson's blood type because the blood already had started to degrade by the time it was tested.
Degraded blood can give a false or misleading test result.
This part of the testimony included graphic photographs of Nicole Simpson's body. This was the first time in several months that the jury had seen photos of the victims, and courtroom spectators said Simpson brushed away tears when the gruesome photos were displayed. During his testimony, Matheson conceded that minor mistakes were made in the investigation. The admission came one day after he defended the work of criminalists Dennis Fung and Andrea Mazzola. The chief forensic chemist told the jury that criminalists should have collected more blood from Simpson's Ford Bronco.