The O.J. Simpson double-murder case took another astonishing twist Friday when a judge aborted a grand jury probe after complaints that unrelenting news coverage may have influenced the panel.

The decision means the case will advance to a preliminary hearing on Thursday, during which prosecutors will have to lay out their evidence in open court, as Simpson's lawyer had wanted.If the grand jury had indicted Simpson first, no hearing would have been held.

The 46-year-old former football star remains in jail on charges of murdering his ex-wife and a friend of hers.

Prominent defense lawyers Alan Dershowitz and F. Lee Bailey have joined Simpson's defense team, and Bailey told CNN's "Larry King Live" they have been working on the case since earlier this week.

A source told The Associated Press that Dershowitz helped on the action that led to Friday's move by Superior Court Judge Cecil J. Mills taking the grand jury off the case.

"As an unanticipated result of the unique circumstances of this matter pending before the 1993-94 grand jury, some jurors have become aware of potentially prejudicial matters not officially presented to them by the district attorney," said Mills, the court's supervising judge.

He told the 23-member grand jury to close its books on the Simpson case. The grand jury had secretly been hearing testimony all week.

Simpson, wearing a dark coat and white, open-collar starched shirt, sat attentively during the hearing and appeared to smile slightly after the decision.

"I don't think I've ever heard of it happening before, but then the publicity concerning O.J. Simpson has been literally unprecedented in magnitude," said John F. Banzhaf III, a law professor at George Washington University who is not connected to the case.

Defense attorney Robert Shapiro and prosecutors had both raised concerns that grand jurors may have been influenced by the publicity.

Since the June 12 slayings, police have released sensational 911 audio tapes, and prosecutors have made several statements about the stabbing deaths of ex-wife Nicole Brown Simpson, 35, and her friend Ronald Lyle Goldman, 25.

There was yet another twist in the case Friday: Deputy District Attorney Marcia Clark disclosed that a blue knit cap was found at the feet of one of the victims. She did not elaborate.

Two days earlier, Clark disputed news reports that a bloody ski mask had been seized from Simpson's home after the slayings. She said no such ski mask existed.

Earlier Friday, Shapiro filed an extraordinary motion in which he asked that the grand jurors be questioned and that the investigation be aborted if any of them had been influenced by news coverage or investigators' statements.

"We feel that the grand jury has been contaminated," Shapiro told Superior Court Judge Lance A. Ito. "It is virtually impossible for Mr. O.J. Simpson to receive a fair trial or a fair review before the grand jury."

In his motion, Shapiro said prosecutors tainted the case when they identified Simpson as the "sole murderer," speculated Simpson might eventually confess, characterized the murders as "a classic domestic violence killing" and authorized the release of the 911 audiotapes.

There were testy exchanges between Shapiro and Clark over use of the media in the case. At one point, Ito asked them to stop.