TUCSON, Ariz. — Attorneys for murder defendant Dr. Bradley Schwartz called two experts Tuesday to counter testimony from prosecution witnesses that DNA found in a murdered doctor's stolen car belonged to the alleged hit man.

Dr. Robin Cotton and Brian Wraxall said that although they did not challenge the science behind the findings of Arizona Department of Public Safety DNA analysts Curtis Reinbold and Lorraine Heath, they did challenge their interpretations of the data.

Prosecutors claim Schwartz hired a hit man to kill a rival ophthalmologist, Dr. Brian Stidham, on Oct. 5, 2004.

Reinbold and Heath previously testified that that there was a high probability that DNA taken from the radio knob of Stidham's white 1992 Lexus came from Ronald "Bruce" Bigger, the alleged hit man.

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Wraxall, however, testified Tuesday that the two prosecution experts falsely assumed that the DNA came from two sources, not three.

"If you make that assumption, then you've got to exclude Mr. Bigger," Wraxall said.

Reinbold said the probability of the DNA coming from a person in the general population other than Bigger ranged from one in 1.9 million to one in 20 million. And Heath said she was confident that the sample did indeed match Bigger's genetic profile.

But Wraxall testified that his analysis of the data found that the likelihood of finding a matching genetic profile was much higher, as low as one in 1,363 for Caucasians.

Cotton, originally hired by the prosecution to analyze DNA evidence from the car and a stolen truck found near the scene of the murder, said she could find no evidence of Bigger's DNA on any of the samples she was given.

On cross-examination, however, prosecutor Sylvia Lafferty pointed out that Cotton was never provided with the DNA mixture found on the radio knob.

Prosecutors allege that Schwartz plotted to kill Stidham out of professional jealousy and personal animosity after Stidham — once an employee of Schwartz's — opened up his own practice across town.

Stidham was stabbed to death in the parking lot outside of his practice, and his car was found about a day later by investigators.

The DNA evidence is key to the prosecution's case. While several witnesses saw a man lingering around the parking lot the night of Stidham's murder, none could identify Bigger in a police lineup.

Schwartz's attorney Brick Storts is expected to continue questioning Wraxall next week after a break in proceedings.

Court TV Extra is streaming the trial live on the Web.