SANTA MONICA, Calif. (AP) _ Mikail Markhasev riveted his gaze on the witness stand as a ballistics expert unveiled the black, long-nosed revolver with which prosecutors claim he killed Bill Cosby's only son.

Criminalist Diana Paul pulled the .38-caliber revolver from an evidence envelope Wednesday and told jurors she had analyzed both it and the bullet that pierced Ennis Cosby's temple.

``The bullet represented here was fired from this Taurus revolver,'' she said.

The police expert, who was with investigators when the rusty gun was recovered from a pile of leaves, was to be cross-examined today.

Her testimony follows a defense attack on evidence gathering, with the defendant's lawyer suggesting that a single minuscule hair tying him to the killing was planted in a dark knit cap.

Investigators found the gun wrapped in the cap on March 7, 1997.

``The gun has some rust on the outside,'' Paul said Wednesday, holding the gun gingerly. ``It was stiff and difficult to open. All the parts were stiff. It just appeared to have been out in the elements.''

It had been nearly two months since the gun was used in the Cosby killing and then discarded, prosecutors say.

Ennis Cosby was shot as he changed a flat tire along a dark road early on Jan. 16, 1997. Shooting witness Stephanie Crane, who had a date with Cosby, told police the assailant wore a cap.

Informants Michael Chang and Christopher So, friends of Markhasev, led police to a brushy area of Coldwater Canyon and the gun was found. Markhasev, 19, was arrested five days later.

But the importance of the cap and gun in the murder case depends on a hair of evidence _ literally. The only physical evidence linking Markhasev to the two items is the strand of hair found in the cap. The hair is 2 millimeters long, or 1/12th of an inch.

Police criminalists say it has Markhasev's genetic markers.

But the defense, borrowing a page from the O.J. Simpson murder trial, attacked evidence gathering and suggested the hair was planted.

Criminalist Susan Brockbank, who also testified in the Simpson trial, said she discovered the single hair 11 months after the cap was found.

Harry Klann, the police department's chief DNA analyst and the one who tested the hair, said it could have come from Markhasev ``or any other individual with the same DNA marker types.''

But he said only one in 15,500 people would have those genetic characteristics. That excludes 99.99 percent of the population, he said.

He added that the hair could not have come from Cosby.

The defense has said it will prove that the true killer is Markhasev's friend Eli Zakaria, who is expected to testify for the prosecution.