FORT HOOD, Texas — A military judge Wednesday threw out Pfc. 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, Lt. Gen. Thomas Metz, who will decide what charges, if any, 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 Capt. Cullen Sheppard said.

The military judge, Col. James Pohl, entered a plea of not guilty for England on a charge of conspiring with Pvt. Charles Graner Jr. to maltreat detainees at the Baghdad-area prison and a related charge.

The mistrial came after Graner, the reputed ringleader of the abuse, testified as a defense witness at England's sentencing hearing that pictures he took of 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 England smiling while standing next to nude prisoners stacked in a pyramid and pointing at a prisoner's genitals.

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

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

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, 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. England, shielded by her defense team, would not comment outside the courtroom.

Allen Rudy, a Dallas attorney, said Wednesday 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," 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 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 Pohl halted Graner's testimony and admonished the defense for admitting evidence that ran counter to England's plea on the conspiracy charge and one count of maltreating detainees.

The judge did not discuss the other five counts to which England had pleaded guilty.

Graner, who is said to be the father of England's infant son, was found guilty in January and is serving a 10-year prison term for his role in the scandal.

In a handwritten note given to reporters Tuesday, Graner had said he wanted England to fight the charges.

"Knowing what happened in Iraq, it was very upsetting to see Lynn plead guilty to her charges," he wrote.

"I would hope that by doing so she will have a better chance at a good sentence."

Graner maintains that he and the other Abu Ghraib guards were following orders from higher-ranking interrogators when they abused the detainees.