The judge in the Fairfax County capital murder trial of sniper John Allen Muhammad is conducting his own investigation of the case in violation of judicial canons of conduct and should recuse himself, prosecutors said yesterday.

Fairfax Circuit Court Judge Jonathan C. Thacher traveled to the Prince William County jail Tuesday, apparently to research the increasingly heated and complicated issue of whether Muhammad's right to a speedy trial was violated when Fairfax authorities made no attempt to bring him to court while he was being tried by Prince William prosecutors.

Prosecutors and legal experts said that it was extremely unusual for a judge to launch his own inquiry into the facts of a case and that the judge should consider only evidence presented by prosecutors and defense lawyers in court. Fairfax prosecutors emphasized that they had asked Thacher to hold a pretrial hearing on the issue he was investigating.

"Judges are not supposed to conduct independent evidentiary investigations," said Steven Lubet, a Northwestern University law professor who specializes in legal ethics. "Personal knowledge of disputed evidentiary facts is definitely a potential basis for disqualification. There are a lot of cases in a lot of states that say judges should not be interviewing witnesses."

During a brief hearing yesterday, Thacher said, "I did go and look at the file yesterday in Prince William County." The judge declined to comment further after the hearing.

Also yesterday, Thacher postponed the trial, which had been scheduled for Oct. 4. He said that with a number of other crucial issues pending, including whether to try the case in Fairfax or relocate it, a trial in less than four weeks "is unrealistic." Thacher rescheduled the trial for Jan. 10.

Thacher, 57, who has been a judge in Fairfax for 16 years, has been closely managing every detail of the case since it was assigned to him early this year. He established a procedure for electronic filing of motions, not typically done in Fairfax, named himself the contact for media issues and has been reviewing all motions before releasing them publicly.

Thacher has taken other unusual steps in the case, including sealing defense billings and bench conference transcripts without a motion from either side. His decision to postpone the trial yesterday was done without a request from either of the parties.

In the past, Thacher has not been hesitant to issue rulings against prosecutors. In 2002, he threw out a confession by Edward Chen -- who admitted he had killed his parents and older brother in Great Falls five years earlier -- saying police had tricked Chen into confessing and denied him a lawyer. On the eve of a potentially close trial, Chen pleaded guilty.

In Muhammad's case, the prosecution and defense have been arguing about his speedy trial rights, a central issue being whether Muhammad was "arrested" on Fairfax's capital murder charges while he sat in the Prince William jail. On Friday, Fairfax prosecutors filed a motion asking Thacher to hold a hearing on the matter.

When the two sides first argued the issue last week, Thacher said he would review the jail's file. But in addition to that, Fairfax prosecutors said, Thacher went to the jail Tuesday and interviewed employees who might be witnesses at a hearing scheduled for Sept. 22.

Deputy Commonwealth's Attorney Raymond F. Morrogh wrote in a motion filed yesterday that he had learned that the judge had "gained personal knowledge of disputed evidentiary facts . . . by speaking to potential witnesses at the jail." The judge also obtained copies of documents that are likely to be argued in the hearing this month, Morrogh said.

And later Tuesday, Morrogh said, Thacher "again spoke to a potential witness at that same facility by telephone." The prosecutor said the judge "discussed disputed evidentiary facts with that potential witness."

Morrogh concluded, "It is not the role of the trial judge to conduct his own investigation or to generate potential evidence on behalf of either side in the case."

Morrogh cited Virginia's Canons of Judicial Conduct, which state: "A judge shall disqualify himself or herself in a proceeding in which the judge's impartiality might reasonably be questioned, including but not limited to instances where the judge has . . . personal knowledge of disputed evidentiary facts concerning the proceeding."

The canons also prohibit "communications made to the judge outside the presence of the parties concerning a pending or impending proceeding" and provide another basis for Thacher to recuse himself, Morrogh argued.

Morrogh's motion was filed moments before yesterday's hearing, and the defense did not respond. Peter D. Greenspun, one of Muhammad's attorneys, declined to comment last night.

Several legal experts were stunned by Thacher's actions.

"That just seems like a most injudicious thing to do," said Michael E. O'Neill, a law professor at George Mason University. "It seems to me that there are legal issues, but those are best presented by the lawyers and not something the judge ought to be doing."

Anne M. Coughlin, a criminal law professor at the University of Virginia, called Thacher's probe "really very unusual. It seems to me that the reading of documents is not that problematic. But with the question of the judge's own probing of a witness, the worry would be that the judge had formed an impression of the witness outside of the courtroom."

Muhammad's attorneys have argued that their client had the right to be brought to Fairfax once he was transported to Virginia for his first trial on capital murder charges in the October 2002 sniper attacks. When Fairfax authorities filed a "detainer" with the Prince William jail, that served as an arrest of Muhammad, and, under the speedy trial rule, his trial should have taken place within five months, the defense contends.

Violation of the right to a speedy trial requires a dismissal of the case that cannot be appealed. Any other Virginia jurisdiction that filed detainers on Muhammad presumably would face the same fate. Cases pending against Muhammad in other states would not be affected.

In an exchange of briefs, prosecutors have said that Muhammad was not arrested on the Fairfax charges until May of this year, after he was sentenced to death in Prince William and shipped to prison in Sussex, Va. The defense then produced Prince William jail documents showing that Fairfax faxed a detainer to Prince William, with a copy of Muhammad's Fairfax indictment and bench warrant, on Jan. 6, and another document showing that it was served on Muhammad on Jan. 9.

Prosecutors answered with a document purporting to instruct the jail not to serve the detainer on Muhammad. That document has not been released by the Fairfax court. The defense said that it did not matter because the detainer was served.

The prosecution argued that staff at the regional jail in Prince William, who are correctional officers instead of sheriff's deputies, do not have arrest powers. The defense replied that they do and that the jail superintendent agreed with the defense.

At that point, Thacher apparently decided to research the matter himself.

"It is not the role of the trial judge to conduct his own investigation," said Deputy Commonwealth's Attorney Morrogh, right, shown in December.