Two days before the start of jury selection in Kobe Bryant's (search) rape case, prosecutors charged that crucial DNA evidence (search) the defense had hoped to use to prove the NBA star's innocence might have been contaminated.

The judge in the case granted prosecutors' request for a hearing Thursday to question the reliability of the defense's DNA experts. The witnesses are expected to argue that the DNA evidence shows the accuser was promiscuous.

A closed hearing will also be held Thursday to discuss the roughly 100-item questionnaire prospective jurors will fill out.

Prosecutors provided no details about the possible contamination in a court filing released Wednesday. But they expressed concerns about defense expert Elizabeth Johnson, who testified in June that DNA evidence suggests the accuser had sex with another man after her encounter with Bryant and before her hospital examination the following day.

The claim is a core part of the defense's strategy to undermine the accuser's credibility and could bolster its claim that she accused Bryant as part of a pattern of behavior intended to gain the attention of a former boyfriend. The woman's attorneys deny the allegation.

The prosecution did not comment on why it waited until so close to the beginning of the trial to raise the DNA issue.

Prosecutor Dana Easter said contamination was found in DNA control samples intended to ensure accurate testing. That is less of a concern than contamination in samples taken from a defendant or accuser, said Scott Robinson, a defense attorney familiar with the case. But he said it could help prosecutors counter the defense expert's theories.

"If prosecutors can demonstrate there's reason to doubt the integrity of the defense lab, that will certainly help," he said. "With DNA technology, that's really about all you can do. Juries are so confused by the science involved, you've got to attack the evidence."

Phil Danielson, director of the University of Denver's Forensic Genetic Institute, said contamination in a control sample could give prosecutors ammunition to argue DNA samples that do not match Bryant could have come from a lab technician, an investigator who gathered evidence, or even a worker at the plant where the testing materials were packaged.

Defense attorneys, who are subject to a sweeping gag order in the case, did not return a call seeking comment.

Bryant, 26, has pleaded not guilty to felony sexual assault, saying he had consensual sex with the woman at the Vail-area resort where she worked last summer. If convicted, the Los Angeles Lakers (search) star faces four years to life in prison or 20 years to life on probation.

During the June hearing, Johnson said her lab and the Colorado Bureau of Investigation both found genetic material matching Bryant and a man identified only as "Mr. X" on the accuser's underwear. Mr. X's genetic material was also found on the woman during the hospital exam, suggesting the two had sex after she was with Bryant, Johnson said.

She said none of Mr. X's genetic material was found on Bryant, again suggesting her encounter with the other man followed her encounter with the NBA star. She said it was unlikely that Mr. X's DNA could have been transferred to her body simply from the underwear.

Johnson's testimony was mistakenly sent to media organizations by a court reporter and later released by the judge after the organizations won a court fight to publish the details.

Prosecutors have known for months that Bryant's attorneys planned to put Johnson on the stand to present DNA test results. Their request for a hearing on the labs' reliability so close to trial was "extremely unusual," former prosecutor Karen Steinhauser said.

"It may be a legitimate issue, but it's something that should have been raised a long time ago," Steinhauser said.

Prosecutors also said some of the data from Technical Associates Inc., the California firm where Johnson works, appears to have been "whited out or otherwise manipulated."

Easter also said defense attorney Hal Haddon had repeatedly refused to provide documents and evidence that should have been shared with the prosecution. Her filing included an Aug. 16 letter in which Haddon told prosecutors much of that material had already been turned over.

"Your letter is a transparent tactic designed to distract us from trial preparation," Haddon wrote.