And a defense witness also retreated from prior statements to police that he saw Hambleton alive after prosecutors say she was killed.

Tim Kupferschmid, lab director for Sorenson Forensics, told a jury of seven men and three women that he misspoke when he testified that more than one man's DNA was found on the fingernails of a girl killed 21 years ago.

"There was potentially, inconclusively some other person's DNA there," Kupferschmid said during cross-examination from the defense. "We couldn't say whether or not it was there."

The revelation of a second person's DNA on 14-year-old Tiffany Hambleton's fingernails surprised prosecutors and defense attorneys Tuesday. It was a critical revelation because the state's case hinges almost completely on new DNA tests that linked Peterson to semen stains and human cells found on Hambleton's fingernails.

Hambleton's partially naked body was found with more than 15 stab wounds in a ditch near Salt Lake City International Airport on March 31, 1986, six weeks after she disappeared. Peterson stands accused of sexually assaulting her, stabbing her to death and dumping her body.

In a fiery volley of questioning, defense attorney Glenn Cook - one of three attorneys representing Peterson, who is 44 - accused Kupferschmid of using outdated equipment and limiting testing in the case to finger only Peterson in the case.

"You ignored [evidence] because didn't come up with results [police] wanted, " Cook said.

Kupferschmid countered that his lab machines are better than most used in government labs - a comment stricken from the record - and that technicians tested the samples with the most potential to match a person's DNA.

The first defense witness to testify in the case, Robert Farrell, told jurors Wednesday he may have been mistaken when he told police in 1986 that he saw Hambleton the night of Feb. 18, 1986. Prosecutors accuse Peterson of sexually assaulting Hambleton and killing her early that morning, hours after meeting Hambleton at a KISS concert.

"It is a possibility that I saw her the Sunday before the KISS concert," Farrell said in cross-examination from Assistant District Attorney Kent Morgan.

Farrell said in an interview after his testimony that the case has "eaten away" at him in the months leading up to trial and that he now doubts his statements to police. He said he may have seen Hambleton the night before she disappeared .

The defense is expected to push a previous police theory by calling the suspect's ex-wife to testify that a member of the Vario Chosen Few street gang was Hambleton's killer.

Peterson is also expected to testify. For 21 years Peterson has denied to police that he had sexual contact with Hambleton the night he met her. He claims he attempted to drive her home but his truck ran out of gas and she walked away.

But defense attorneys this week indicated that they will not dispute prosecutors' claims that Peterson had sexual contact with the teen. Peterson was arrested and charged with first-degree felony murder in January, after Sorenson Forensics, based in South Salt Lake City, used new DNA technology to match evidence collected in 1986 to Peterson.