The wooden floor of the bedroom where Montgomery County prosecutors say Michele Dorr died under Hadden Clark's knife went on display yesterday in a Rockville courtroom with more than 35 bright yellow marks where prosecutors say blood seeped into the floor's grooves.

The dark brown varnished floor, spanning 8 feet by 10 feet, was taken from the second-floor bedroom of the Silver Spring home owned by Clark's brother. Prosecutors say Michele, 6, who was last seen playing in her father's back yard two houses away, wandered into that bedroom looking for Clark's niece, her favorite neighborhood playmate, on the day she vanished in May 1986.

Prosecutors had turned the floor on its side and propped it onto a large easel with wheels, allowing jurors in Clark's murder trial to view it like an 80-square-foot painting. Susan Ballou, a forensic scientist in the Montgomery police crime lab, pointed out the bright yellow lines in the grooves between the wooden slats, where she said she swabbed more than 100 places where blood remained 10 years after Michele disappeared.

The yellow lines in the grooves clustered near what would have been the bedroom door and in the middle of the floor, which Ballou said would have been in front of a day bed that was in the room in 1986. She told the jury that the remnants of blood glowed in the dark when she sprayed the floor with a chemical that reacts with blood.

Three of Clark's former prison inmates have testified that Clark told them he discovered Michele playing with dolls in his niece's bedroom before he slashed her across her chest with a butcher knife and then nearly decapitated the child with a cut to her throat.

The inmates said Clark boasted of scrubbing the floor clean of the child's blood before carrying her body out of the house in a duffel bag. Her body has never been found.

During the prosecution's presentation, Clark, 47, gazed at the wooden floor with his brow furrowed slightly, as if he were studying a portrait, and then turned away to gaze impassively at the defense table.

Ballou also testified that she found blood beneath tape wrapped around the five-inch handle and part of the six-inch blade of a filet knife found among Clark's possessions in a rented storage locker in Rhode Island. She said she also found blood on three pocket knives seized from the storage bin.

Prosecutors argued in pretrial hearings that highly sensitive DNA tests showed that the blood beneath the tape on the filet knife handle was likely from Michele. However, the jury won't hear that. Montgomery Circuit Court Judge Michael D. Mason ruled that the DNA test results were not conclusive enough to be helpful to jurors.

Defense attorneys are scheduled to cross-examine Ballou on the blood evidence this morning.

Jurors also heard yesterday from a Prince George's County private investigator who said she was Clark's pen pal for two years while he was in prison. Clark is serving a 30-year sentence for the 1992 murder of Laura Houghteling, 23, of Bethesda. The jury has been told only that Clark is in prison, but not why he is there.

Sharon Weidenfeld, of Greenbelt, testified that she wrote to Clark in 1996 asking him to reveal the location of Michele's body.

"I believe in God," Clark wrote back in a letter Weidenfeld read aloud in court. "He and I know I have done a terrible thing. . . . I know I've done a terrible thing, and I'm sorry for doing it."