ANN ARBOR, Mich. — A defense expert acknowledged Thursday that DNA on the pantyhose of a law-school coed killed 36 years ago belongs to the man on trial for her murder. But the molecular biologist testified that the mysterious presence of DNA from a second man not involved with the crime suggests lab contamination occurred.

"Knowing how it is that DNA transfer can occur, knowing how little material needs to be transferred ... it certainly seems to be a very improbable coincidence to me that contamination did not occur," the expert, Dan Krane, told jurors.

Krane, an associate professor of biological sciences at Wright State University in Dayton, Ohio, was the final witness in the trial of retired nurse Gary Leiterman.

The 62-year-old faces life in prison for the 1969 shooting of Jane Mixer, a University of Michigan student who went missing after accepting a ride home for spring break with a stranger.

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Leiterman, who had no known connection to the victim, was arrested on the strength of DNA tests that found his genetic profile in three stains on Mixer's pantyhose.

Krane's testimony echoed defense claims that the DNA evidence is meaningless because of other tests at the state police crime laboratory in Lansing that matched a spot of blood on the victim's hand to a convicted murderer who was just 4 years old at the time of the killing.

Bloody clothing belonging to the man, John Ruelas, was being tested at the lab during the same time period analysts were examining items from the Mixer murder. Lab officials have denied contamination occurred, but police detectives have not established any connection between Ruelas and his relatives and Mixer.

Krane conceded that the lab had safeguards against contamination and said nothing in the lab records indicated that contamination had taken place. But he said, the confounding presence of Ruelas' genetic material appeared to leave few other explanations.

"It just strikes me as a remarkable coincidence, not necessarily an impossible coincidence, but a remarkable one," he said.

Krane was one of five witnesses who testified in the defense case Thursday. A handwriting analyst contradicted the testimony of a police expert who matched the defendant's cursive to two words scrawled on a phonebook in the basement of Mixer's dorm.

The defense expert, Robert Kullman, said his analysis indicated it was "highly probable" Leiterman did not write "Mixer" and "Muskegeon," a misspelling of the victim's hometown.

"It is absolutely different," Kullman said.

The defense also called as witnesses three people who knew Leiterman at the time of Mixer's murder. Two said they did not recall Leiterman having a shooting range in the basement of his house. A witness for the prosecution, Paul Esper, said Leiterman had such a range and used it to practice shooting a gun similar to the one that killed Mixer.

A friend, Trudie Messingham, said Leiterman was always friendly and kind.

"Ever notice any changes in his mood or demeanor during this time that stuck out?" defense attorney Gary Gabry said.

"No," she replied, shaking her head.

Jurors are slated to begin deliberations Friday. The trial is being streamed live on Court TV Extra.