DURHAM, N.C. — The prosecution's chief expert on blood spatter will not take the stand at novelist Michael Peterson's murder trial until August. But that isn't preventing his defense lawyers from already trying to undermine the conclusions that expert may offer.

The prosecution is counting on its blood spatter expert's interpretation to bolster its claim that Peterson beat his wife to death in a stairwell and staged the scene to make it appear a fall. Meanwhile, the defense has been working to persuade jurors that evidence collection by police was so flawed that any conclusions the expert might reached about how Kathleen Peterson died are all but meaningless.

Toward that end, defense lawyer David Rudolf asked crime scene technician Eric Campen on Monday whether the blood-illuminating chemical luminol, or some other liquid, was used on the wall above the main blood stain in the stairwell where Kathleen Peterson's body was found on Dec. 9, 2001.

Rudolph pointed to photos of the section of wall in question. An oval area was free of blood (see picture above), but some kind of liquid appeared to have dripped down from what Rudolph call a "void" area into the main blood stain. Campen remained adamant that neither he nor any police officer applied luminol to the wall in the stairwell, and wouldn't need to because there was ample blood visible.

"Does that have the appearance luminol would look like after it was sprayed and dried?" Rudolf said, pointing to the streaks on the wall.

"To some extent, yes," Campen agreed. "I don't know what made those marks on the wall ... We never sprayed luminol in that area."

So how did the streaks get there if police did not spray the wall with luminol? Could Peterson have tried to clean the wall before calling 911 and decided against continuing?

Michael Peterson listens to testimony Monday.

The defense is expected to argue that the streaks were caused by luminol and not by Michael Peterson trying to clean up blood before calling for an ambulance. Curiously, other police photos taken after Michael Peterson was removed from the area of the stairwell do not show the streaks.

Campen insisted that no one from the police department besides himself or people he supervised used luminol at the crime scene. Rudolf seemed to mock Campen's testimony that after spraying luminol on the floor of the kitchen, he observed the outline of human toes and found a faint trail of bloody footprints that lead to two different sinks.

Prosecutor Jim Hardin Jr. didn't even to try to rehabilitate the state witness on redirect examination. He let Campen leave the stand without addressing the issues Rudolf raised during a thorough cross-examination.

Outside of the jury's presence, Rudolf raised an issue about a computer-generated diagram that Campen had created, but later deleted, to show the trail of apparent footprints. Campen had been under a court order to produce the diagram but had decided that the diagram was "misleading" because it showed shoeprints rather than footprints.

Judge Orlando Hudson Jr. reserved decision on whether to instruct jurors that they may consider the missing evidence a weakness in the prosecution's case, as the defense requested.

When testimony resumes at 9:30 a.m. on Tuesday, jurors will view photographs of the crime scene taken by evidence technician Angie Powell.

The trial, now in its fifth week, is being broadcast by Court TV.