All around the Fairfax County courthouse, lawyers, deputies and clerks were debating Circuit Court Judge Jonathan C. Thacher's bold move last week from jurist to investigator in the John Allen Muhammad sniper case.

The verdict was unanimous: What was he thinking?

As soon as Fairfax prosecutors learned that Thacher had traveled to Prince William County on Tuesday to interview jailers and review jail files, they asked Thacher to step down from the Muhammad trial. Thacher has not ruled on the request and declined to comment on his actions.

Thacher's actions were surprising because he is not regarded as rash or reckless by lawyers who know him. He often bats around legal issues with other judges or lawyers and has spent many hours researching cases and theories to resolve thorny questions in the complicated sniper trial.

Thacher, 57, is a well-traveled lawyer and athlete with a colorful life away from the bench: a black belt in tae kwon do, a builder (and player) of electric guitars, a one-time high school basketball coach, a former world class water-skier. Before becoming a full-time judge in 1994, he spent years as a criminal investigator in the military and then worked as a criminal defense lawyer in private practice.

"He's one of the hardest-working guys I've ever known in my life," said James E. Swiger, a former law school classmate and law partner of Thacher's. "I think he views this [Muhammad] case as one of the most important he'll ever work on, and he's making sure he does everything just right."

But the motion to disqualify Thacher threatens to derail a case that had been chugging steadily toward a second death penalty trial for Muhammad, 43, who was convicted of one set of capital murder charges in Prince William last year.

As one of many pretrial challenges, Muhammad's attorneys claimed that the sniper's right to a speedy trial had been violated. Virginia law requires that a jailed defendant be tried within five months. Even though Muhammad was being prosecuted first in Prince William, charges were pending in Fairfax at the same time, and his attorneys claimed that Muhammad had the right to be brought to Fairfax and possibly choose to move quickly to trial there.

Fairfax prosecutors argued that Muhammad was not arrested on the Fairfax charges until May of this year, and he was brought to Fairfax in June. Muhammad's attorneys responded with documents showing that Fairfax sent a "detainer," or arrest authority, to the Prince William jail in January, and it was served on Muhammad.

Thacher said in a hearing that he didn't think either side had provided him all the documents in the case. So Tuesday, he went to the Prince William courthouse, met with jailers and reviewed jail records, according to prosecutors.

What outraged the Fairfax commonwealth's attorneys was that, after the flurry of arguments and counterarguments on the speedy trial issue, they had asked for an evidentiary hearing four days before Thacher's research trip. Deputy Commonwealth's Attorney Raymond F. Morrogh, in asking Thacher to recuse himself, said the judge might well have interviewed witnesses who later would appear before him.

Judges typically confine themselves to information and argument presented only by the parties to a lawsuit or criminal case, to avoid any claim that they considered irrelevant or tainted facts. Judges always advise jurors not to investigate a case or listen to anything about it outside the courtroom.

The Canons of Judicial Conduct in Virginia instruct that judges shall disqualify themselves if they have "personal knowledge of disputed evidentiary facts." And the commentary section of the canons, taken from previous rulings, states: "A judge must not independently investigate facts in a case and must consider only the evidence presented."

Legal experts said Thacher's actions were unusual at best and wrong at worst. "It seems odd that the judge would go over there and do fact-finding himself," George Mason University law professor Michael E. O'Neill said last week. "Was there a legal reason, or a justifiable reason, for the judge to be there? Right offhand, I can't think of one."

Thacher is not considered either a pro-prosecution or pro-defense judge by lawyers in the courthouse. Some defense lawyers said they felt he gave them their best shot at winning cases. "He's always been very fair with me," one defender said.

Others said they dreaded appearing in front of him. "He thinks he's smarter than everyone," one defense lawyer said. Prosecutors voiced similar opinions, both good and bad. No prosecutors or defenders would speak on the record, even those who like the judge, for fear of offending Thacher or his colleagues. None believes Thacher will recuse himself from the case.

Thacher's background is, in many ways, typical of a military upbringing. Born in Boston in 1947, the second son of a World War II veteran, he grew up in Fort Leavenworth, Kan.; Turkey; Arlington; Taipei, Taiwan; California; and the Philippines, with some of his school years spent at Stratford Junior High and Washington-Lee High schools in Arlington. He earned a bachelor's degree in history and English from the University of Miami in 1970.

Thacher entered the Army after college. He trained in the infantry but wound up in Thailand instead of Vietnam. He later transferred to the Army's Criminal Investigation Division, which investigates criminal behavior by military personnel. When Thacher left the Army, he joined the Navy's investigative service and did three more years of investigative work, much of it in narcotics.

At the same time, at night, Thacher began attending George Mason's law school. After he earned a law degree in 1980, at age 33, he went into private practice while working as minority counsel on a subcommittee chaired by then-Sen. Robert J. Dole (R-Kan.) investigating alleged lobbying by President Jimmy Carter's brother, Billy Carter, on behalf of Libya.

Thacher began serving as a substitute judge in Fairfax General District Court in 1988. He was appointed a full-time district court judge in 1994 and was elevated to the circuit court in 1998. Circuit court judges serve eight-year terms, and he will be reviewed for reappointment by the area's state legislators in 2006.

But Thacher hardly focused all his time on law. He took up water-skiing while in college and competed in tournaments for years. The judge is also a hoops junkie and was the freshman basketball coach at Paul VI Catholic High School in Fairfax until 2001 under coaching legend Red Jenkins.

Thacher also builds electric guitars and plays a little old-school rock-and-roll. And he speaks Mandarin Chinese.

Before the Muhammad case, Thacher's most high-profile case was the triple murder prosecution of Edward Chen of Great Falls in 2002. Chen had shot and killed his parents and brother five years earlier and disposed of their bodies in the Chesapeake Bay. The crime was discovered when, out of the blue, his former girlfriend notified police. Confronted, Chen confessed.

Early in the confession, after Chen signed a waiver of his Miranda rights, Detective Steve Shillingford asked Chen why he had killed his family. Chen said, "I don't want to talk about it." Shillingford then asked how Chen did it, and Chen related the crimes in crisp detail.

But Thacher ruled that Chen's reply to the question about his motivation was "an invocation of his right to remain silent." Prosecutors were appalled, and a hard-fought trial loomed before Chen pleaded guilty on the eve of jury selection.

The Chen ruling has prosecutors fearing that Thacher again might side with a violent defendant, this time on the speedy trial issue.

And a dismissal for violation of the speedy trial right cannot be appealed.

Swiger, Thacher's former law partner, said the judge is not as deferential to prosecutors, "and I think a lot of that is what the commonwealth is fuming over now.

"I think some of the criticism is ill-deserved," Swiger said, "because he acted with the utmost integrity."

Jonathan Thacher was asked to recuse himself.