Gregory Scott Hopkins' DNA was found on the back of Catherine Janet Walsh's nightgown, on the rope that bound her hands and on the sheet that covered her because the two had sex in her bedroom three to four weeks before her death, Mr. Hopkins' attorney has argued.
But on Friday, Cyril H. Wecht testified that he disagreed with that scenario.
Dr. Wecht, who has been a forensic pathologist for more than 50 years, said that after reviewing the crime scene photographs, the autopsy and other materials from the 1979 case, he considered it "hard to believe" that Mr. Hopkins' DNA was found in those specific places due solely to an earlier sexual encounter.
"I just find that extremely unlikely," he said.
Friday was the fourth day of testimony at the Beaver County Courthouse in the trial of Mr. Hopkins, 67, a former Bridgewater, Beaver County, councilman who is charged in the Sept. 1, 1979, homicide. On that day, Walsh's parents discovered their daughter on the bed in her Monaca home, lying facedown and covered by a sheet.
She had been strangled to death with a blue handkerchief, and her hands were bound behind her wrists with rope from a bathrobe. There was no indication of recent sexual activity. With no DNA testing available then and little additional evidence to pursue, no charges were filed in the case until January 2012, when Mr. Hopkins was arrested for criminal homicide after DNA from seminal fluid found in three places matched a sample taken from him.
James Ross, the attorney representing Mr. Hopkins, has argued repeatedly that Mr. Hopkins' DNA should have been found in Walsh's bedroom, since the two had sex in her bed a few times. On Friday, when the Beaver County assistant district attorneys trying the case called witnesses to testify about DNA discovered, he pressed on that point.
Ashlee Mangan, a forensic scientist at the Pennsylvania State Police's Greensburg crime lab, described finding "an abundant amount of sperm" on the nightgown, the top sheet and the rope. Ms. Mangan said she doubted that much semen would be present if the items had been washed. She found small amounts of semen on the fitted sheet and the bathrobe.
Mr. Ross asked Ms. Mangan to make clear to the jury that her examination could not explain when or how the semen was deposited, just that it was present. He also questioned why Ms. Mangan tested for only semen when the alternate light source found other stained areas on the items.
"What if other spots had saliva, for example, pointing to another suspect?" he asked.
Toward the end of his questioning of Dr. Wecht on Friday, Mr. Ross asked whether it was possible that Walsh may have perspired because she was being strangled and that her sweat could have turned an old semen stain on her nightgown wet, causing the DNA there to transfer onto the rope around her hands and the top sheet.
"I just can't envision it," Dr. Wecht said.
The discussion of DNA evidence will continue when the trial resumes Monday. Mr. Ross has said he plans to bring as a witness Mark W. Perlin, the CEO of Cybergenetics, who will discuss a DNA analysis he conducted that showed DNA from an unknown person was found on Walsh's bathrobe, and that DNA from Walsh's estranged husband, Scott Walsh, was found on the fitted sheet.
Kaitlynn Riely: firstname.lastname@example.org or 412-263-1707.