DURHAM, N.C. — It might not have been a fireplace "blow poke," but something light and cylindrical was used to beat Kathleen Peterson to death, an injury biomechanics expert testified Wednesday in Michael Peterson's murder trial.

"The injuries, lacerations, bruises, and contusions in my mind are inconsistent with a fall down the steps and are consistent with a beating with a blunt instrument, most likely a rounded blunt instrument," testified James McElhaney, an emeritus professor of engineering at Duke University, during the prosecution's rebuttal case.

An analysis of Kathleen Peterson's injuries

McElhaney was brought in to back up the prosecution's claim that Peterson bludgeoned his wife of five years to death in the steep rear stairwell of their massive North Carolina home with something like a fireplace implement missing from the Peterson kitchen. Prosecutors believe that the novelist disguised the murder as an accident.

Peterson, 59, says he was out by the pool when his wife must have fallen on the stairs.

McElhaney's testimony came a day after Peterson's defense rested after dramatically unveiling what they claim is the blow poke the prosecution had suggested as the murder weapon.  The fireplace poker, which the defense said it found in Peterson's garage a few days ago, did not appear to show signs it had been used in a beating.

The avuncular professor was brought in to refute the testimony of the defense's injury biomechanics expert, Faris Bandak, who analyzed the victim's injuries and concluded they were accidental.

McElhaney, however, said that six facets of the injuries led him to conclude that they were beating-induced: The location, length, number and orientation of the injuries, and the velocity and amount of energy that went into the seven lacerations on the back of Kathleen Peterson's head all ruled out a stairway fall.

"When I put all of these things together I conclude that [with] the lack of skull fracture and brain injury ... this was a relatively light object striking the head [at] the maximum velocity for laceration," he said, not "a fall down the steps ... which I would also expect to cause brain injury and skull fracture."

McElhaney was the fourth rebuttal witness asked to testify by prosecutors, and retook the stand under cross-examination by David Rudolf, Peterson's defense attorney.

But while other experts were able to stick to their conclusions against cross-examination from the defense attorney, McElhaney gave Rudolf some room to turn his testimony against the prosecution.

Though he called it a "stretch," the professor admitted it was possible for Kathleen Peterson's head to have been lacerated by falling against the doorway molding and stairs, for her to have fallen down twice, and for each fall to have caused multiple lacerations. All of those possibilities fit the defense's accident scenario.

Meanwhile, prosecutors aren't giving up on their blow poke theory.

Lori Hunt Campell, the victim's younger sister by nine years, also testified Wednesday that she recalled seeing the blow poke next to the fireplace in the Peterson's kitchen in July 2001, only five months before her sister died.

Lori Hunt Campell, Kathleen Peterson's younger sister

Campell broke into tears as she recalled visiting her sister for the last time with her mother and her sons William and Edward. The blow poke seemed to catch the attention of 5-year-old Edward, she testified, and she had to tell him to leave it alone.

"He likes to play with sticks and swords and guns.  I had to tell him 'stop playing with it, you're going to break something,'" said Campell. "Kathleen's house was not babyproofed."

Unlike the dingy, corroded blow poke produced by Peterson's defense Tuesday, the one she recalled was shiny and unused.

Campell was taken aback when police failed to find the tool during a scan of the property after Kathleen Peterson's Dec. 9, 2001, death.

"We all wondered what had happened to it," said Campell.

Without the missing blow poke —which may or may not be the one that surfaced Tuesday — prosecutors elected to use a similar one for demonstrative purposes, supplied by another sister, Candace Zamperini, who gave Kathleen Peterson the missing fireplace tool in 1984.

Rudolf has said that prosecutors attempted to hide this blow poke, as well as the entire blow poke theory, by failing to bring it out for his investigators and experts during several visits to the property room in the Durham police department.

To rebut cover-up claims by the defense, prosecutors called a property custodian from the Durham police department Wednesday.

Custodian Ruth Brown insisted that the blow poke provided to police by Zamperini was in the property room when Rudolf visited.

But on cross-examination, Brown admitted that some records from the time made no mention of the item.

Peterson's trial, which is being televised by Court TV, will resume at 9:30 a.m. on Thursday.