BARNSTABLE, Mass. — Investigators probing the rape and murder of fashion writer Christa Worthington failed to test a key piece of evidence, a defense witness testified Thursday.

The evidence — a swab from outside Worthington's body that tested positive for the presence of semen — was one of four pieces of evidence that should have been analyzed in the months after the January 2002 killing, said Richard Saferstein, a forensic expert who has written and lectured extensively on the subject.

"I think there were some very serious omissions," said Saferstein, former head of the New Jersey State Police crime lab.

Sperm found inside the 46-year-old victim's body matched defendant Christopher McCowen's DNA. The 34-year-old garbage collector told police he had consensual sex with Worthington days before the killing; later, he said a friend named Jeremy Frazier plunged a knife in her chest.

When McCowen had sex with Worthington, and whether it was consensual, are central issues at his trial on charges of first-degree murder and aggravated rape.

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The swab that Saferstein insisted should have been analyzed for DNA was taken from outside the vaginal cavity. Saferstein explained that semen can be present internally for days, but semen deposited outside the cavity likely was fresh.

"If the external swab was not Christopher McCowen's, that would mean the defendant most likely would have been exonerated," Saferstein said.

Pressed later by Assistant District Attorney Robert Welsh III, Saperstein backed off that statement. "It certainly would have changed the complexion of this case," Saferstein said.

The prosecutor did not ask why the swab wasn't tested for DNA, but did use Saferstein's own textbook on criminalistics to get the defense witness to agree that it is not practical to test everything found at crime scenes.

As for other items that were not tested, including hair, fabric fibers, blood and footprints, Welsh noted that paramedics placed a blanket over the body and that Worthington's toddler daughter, Ava, tried to clean the body before an ex-boyfriend came across the gruesome scene.

Police focused their attention on Worthington's ex-boyfriend and others before finally matching the DNA to McCowen. He claims that Frazier killed Worthington because she confronted him about taking items from her beach house, but McCowen says he was intoxicated at the time and doesn't remember the details clearly.

Worthington's cousin testified Thursday that Worthington would have told her if she was romantically involved with anyone. Called to testify for the defense, Pamela Worthington-Franklin said she was in shock and does not recall telling police the day after the killing that Worthington was not concerned about a man's looks, social, economic or marital status.

"I don't recall saying those words," she said.

Defense lawyer Robert George appeared to be trying to counter his fear that jurors could convict McCowen if they believe a sophisticated, wealthy white woman would never consent to sex with a black garbage collector with an IQ below 80.

Jurors also heard Thursday from two defense witnesses about Jeremy Frazier, the man McCowen tried to implicate as the real killer. One witness was called to shake Frazier's alibi that he was at a friend's house that weekend because the friend's father was away.

Another witness said that Frazier called him a few months before the murder to say that he needed a quick "pickup," or score, and that he knew of a woman whose home might contain cash and jewelry. Welsh, the prosecutor, described the testimony as "nebulous."

The defense also called a psychologist to testify about the role McCowen's below-average intelligence could have played in the April 2005 police interview that led to his arrest.

After five meetings with McCowen, Dr. Eric Brown concluded that McCowen's IQ is 78. Given the amount of stress placed on him by the situation, and the length of his police interview, McCowen's ability to resist manipulation was "severely compromised," Brown surmised.

"In the best of conditions, Mr. McCowen lacks a good understanding of language and conversation," Brown said. "He is likely to misinterpret. Having a low IQ like that also makes him very susceptible to manipulation, making him susceptible to agreeing to what others want him to think and say."

If convicted, McCowen faces life in prison without parole.

The trial is being broadcast on the web by Court TV Extra.