DURHAM, N.C. — A lawyer for novelist Michael Peterson worked methodically and aggressively Tuesday to challenge a blood spatter expert who testified that he believes Peterson used a weapon to beat his wife to death in the stairwell of their home.

Testifying for a fifth day, North Carolina Bureau of Investigation Agent Peter "Duane" Deaver did not want to concede much during cross-examination by David Rudolf, a defense attorney known for remaining patient until he gets what he wants from witnesses. At times Rudolf seemed to succeed in making Deaver testy.

Rudolf didn't get Deaver to change his opinion that Kathleen Peterson was beaten to death, but he did give jurors information they could use when they assess how much weight to give to Deaver's opinion.

Rudolf sought to persuade jurors that the blood spatter analyst made mistakes, conducted scientifically unreliable experiments, skewed tests to help the prosecution and ignored facts that might contradict his conclusions (see report).

In questions that sounded more like statements, Rudolf suggested that someone wielding a fireplace poker in the stairwell where Kathleen Peterson died probably would have hit the door frame with the weapon. Prosecutors have said they believe a missing fireplace poker — or something like it — caused seven distinct lacerations on the back of Kathleen Peterson's head before she was found dead on Dec. 9, 2001.

Rudolf also mocked tests Deaver conducted in a $7,700 replica of the Petersons' staircase the prosection had built. Deaver said the tests were conducted to try to explain stains, but the defense maintains that the replica was really built so that investigators could try to recreate what police believe was a murder.

The defense attorney began his cross-examination of Deaver by revisiting Deaver's Monday testimony that he had not seen any fireplace implements at the Peterson home on Dec. 9, 2001, but had noticed some when he returned June 27, 2002, to take measurements.

Rudolf showed Deaver two ornamental walking canes and the witness identified them as the items he thought were fireplace tools during his 2002 visit to the house. Rudolf then projected a photograph and video still on a screen that appeared to show that the walking canes were near the fireplace when police photographed it on the night of the death.

Deaver said he could not recognize the canes in the projected photograph, so Rudolf approached the witness stand and gave him the photo to hold and examine. Still no luck.

Rudolf then returned to the defense table and retrieved a magnifying glass for the witness.

When Deaver said he still couldn't be sure that what he was seeing was the canes, Rudolf gave the photo and the magnifying glass to each juror to examine for themselves.

Why Deaver was unable or unwilling to identify the canes in the picture is unclear.

The blood stains Deaver observed and what they mean, however, is a central issue jurors will have to grapple with. Deaver never wavered from his opinion that a line of five blood drops found outside the stairwell and well above the floor was made by someone swinging an object with blood on it back in an arc.

Deaver said that the the point in space where that object was swung down onto a bloody target was away from the walls and higher than the steps. That fact lead him to conclude that the cast-off blood stain could not have come from Kathleen Peterson striking her head on the steps or wall in a fall.

However, there were other cast-off stains that Deaver could not associate with two other points of impact he identified from the blood spatter on the walls. Such blood spatter is produced by an object actually striking a bloody target.

He also could not offer a theory of how a fireplace poker or similar object would not have hit the overhead molding on the door frame leading into the stairwell if someone had been wielding such a weapon from that spot, as Deaver testified.

Noting that Peterson is 5 feet 9 inches tall and the door frame is a foot higher, Rudolf suggested that a long-handled weapon almost certainly would have come into contact with the frame and left a mark.

"You think it might catch on this header here?" Rudolf asked, referring to the door frame.

"It could," Deaver said.

Deaver apparently anticipated such questions during tests he conducted inside the stairwell replica. During the tests, he choked down on bars used to impact bloody wigs and sponges in an effort to recreate stains found in the actual stairwell.

The amount of blood in the stairwell and the location of blood, particularly outside the stairwell, are key components of the prosecution's case. The defense claims that the cast-off stains outside the stairwell were created when a distraught Michael Peterson had to forcibly be separated from the body of his dead wife.

In another statement in the guise of a question, Rudolf asked Deaver whether he knew that Henry Lee, the world-famous forensic scientist from Connecticut, had reviewed Deaver's work and concluded that the scene was not consistent with a beating.

With no objection raised by the prosecution, Deaver simply said he had not.

Deaver will be on the witness stand again when testimony resumes at 9:30 a.m. Wednesday. At 3 p.m., when jurors are scheduled to be released, Judge Orlando Hudson Jr. will continue a hearing about the admissibility of evidence the prosecution says it has that Michael Peterson was involved in a similar staircase death in Germany involving a neighbor in 1985.

The trial is being broadcast by Court TV.