FORT HOOD, Texas – A military judge yesterday threw out Private First Class Lynndie England’s guilty plea to abusing Iraqi detainees at Abu Ghraib prison, saying he was not convinced the Army reservist who appeared in some of the most notorious photos in the scandal knew her actions were wrong at the time.

The mistrial marks a stunning turn in the case and sends it back to square one.

The case will be reviewed again by Fort Hood’s commander, Lieutenant General Thomas Metz, who will decide what charges, if any, Private England should face. If she is charged, the case would go back to a military equivalent of a grand jury hearing, an Article 32 proceeding, prosecution spokesman Captain Cullen Sheppard said.

The military judge, Colonel James Pohl, entered a plea of not guilty for Private England on a charge of conspiring with Private Charles Graner Jr. to maltreat detainees at the Baghdadarea prison and a related charge.

The mistrial came after Graner, the reputed ringleader of the abuse, testified as a defense witness at Private England’s sentencing hearing that pictures he took of Private England holding a naked prisoner on a leash at Abu Ghraib were meant to be used as a legitimate training aid for other guards.

Other photos showed Private England smiling while standing next to nude prisoners stacked in a pyramid and pointing at a prisoner’s genitals.

Private England maintained the same stoic look she has had throughout the proceeding. During a recess before the plea deal was thrown out, Private England peeked at a sketch artist’s drawing of Graner on the stand. “Don’t forget the horns and the goatee,” she said.

When Private England pleaded guilty Monday, she told the judge she knew that the pictures were being taken purely for the amusement of the guards.

Judge Pohl said her statement and Graner’s could not be reconciled.

“You can’t have a one-person conspiracy,” the judge said before he declared the mistrial and dismissed the sentencing jury.

Under military law, the judge could formally accept her guilty plea only if he was convinced that she knew at the time that what she was doing was illegal.

By rejecting the plea to the conspiracy charge, Judge Pohl canceled the entire plea agreement. The agreement had carried a maximum sentence of 11 years in prison, but the prosecution and defense had a deal that capped the sentence at a lesser punishment; the length was not released.

Neither prosecution nor defense lawyers would speak to reporters after the deal was discarded. Private England, shielded by her defense team, would not comment outside the courtroom.

A Dallas attorney, Allen Rudy, said yesterday he could not recall a military plea being scrapped under such circumstances during his 25 years as a Navy lawyer and judge.

“That is a shocker,” Mr. Rudy said. “But [Pohl] has to protect the defendant in that situation. … He has to make sure [England] wasn’t talked into it by her lawyer or her parents or someone else.”

During defense questioning, Graner said he looped the leash around the prisoner’s shoulders as a way to coax him out of a cell, and that it slipped up around his neck. He said he asked Private England to hold the strap while he took photos that he could show to other guards later to teach them this prisoner-handling technique.

At that point, Judge Pohl halted Graner’s testimony and admonished the defense for admitting evidence that ran counter to Private England’s plea on the conspiracy charge and one count of maltreating detainees.