A vigorous cross-examination didn't shake the confidence of a medical examiner who maintained Friday that the death of novelist Michael Peterson's wife was a clear homicide, not the result of a fall.

Prosecutors wrapped up their 40-day, 50-witness case with assistant North Carolina medical examiner Deborah Radisch.  She rebuffed suggestions by David Rudolf, Peterson's attorney, that her conclusion, based on an autopsy of Kathleen Peterson, was flawed.

"In order to rule out a fall you have to rule out a freak fall, you have to rule out falls that are unusual, you can't just rule out a tumble down the stairs front-wise," said Rudolf, "because people fall in all kinds of different ways, right?"

"Yes," replied a skeptical Radisch after a long pause, scrunching up her face and shaking her head in mock confusion.

Peterson claims that on the night of Dec. 9, 2001, he came inside their Durham, N.C., home after spending 45 minutes by the pool to find his wife laying in a pool of blood at the bottom of a staircase.  But the amount of blood at the scene, and a pathology report that later showed seven lacerations on her head, lead police to conclude that her death was not an accident. 

Charged with first-degree murder, the novelist could spend life in prison if convicted.

On Thursday, Radisch testified that Kathleen Peterson's death was a blunt trauma homicide, i.e. that she was beaten to death.  Radisch pointed to the series of distinct lacerations on the victim's head, calling them too numerous to have come from a tumble down a narrow and dimly lit stairwell. Radisch said defensive wounds on her hands and broken cartilage in her throat, indicating a strangulation attempt, also supported her finding of a homicide.

She also described her analysis of hundreds of other fatal falls in North Carolina since 1991, none of which produced as many as the seven deep lacerations she found on the back of Kathleen Peterson's head.

The medical examiner's involvement in the Peterson's case goes beyond the death of Kathleen Peterson, to a similar death of a neighbor in 1985, when Michael Peterson lived in Germany. 

On Friday, Radisch was also cross-examined on her analysis of the body of Elizabeth Ratliff, the neighbor who died in an apparent stairway fall. The previous evening Peterson had walked the family friend home and was the last person to see her alive.

Radisch, who examined Ratliff's exhumed body in April, disputed the medical analyses of German police, a U.S. Army pathologist and a distinguished pathology institute that autopsied Ratliff shortly after her death.

Those authorities had concluded that Ratliff had fallen after suffering a brain hemorrhage.

"I believe they were wrong," she said Friday, responding to skepticism from Peterson's lawyer.  "I think she died of an intercranial hemorrhage, but I don't think it was a spontaneous intercranial hemorrhage."

Radisch fended off the lawyer's questioning with a dry wit, raised eyebrow, and, at times, outright hostility.  

Defense lawyer Rudolf (left) and Michael Peterson during the trial Friday

"Just because you don't agree with a diagnosis doesn't mean it's wrong, does it?" the attorney asked, referring to her analysis of Ratliff's death.

"Doesn't mean that it's wrong," she said, "but I've spent almost a year an a half researching similar diagnoses ... and I haven't found another case that this has been the diagnosis."

The state's brief re-direct closed by asking Radisch whether she still believed her analysis was correct.

"Do you still hold the opinion that the manner of death with respect to those injuries was due to homicide?" asked prosecutor Jim Hardin.

"Yes I do," she said.

After the prosecution rested its case at the end of the day, Peterson's lawyer said he will announce Monday whether he plans to call witnesses for the defense. 

Peterson's trial, which is being broadcast by Court TV, is scheduled to resume on Monday at 9:30 a.m., ET.