Spc. Jeremy Sivits's tearful apology and no-excuse testimony at his court-martial on Wednesday will make him a credible witness against other soldiers charged with mistreating Iraqi detainees at Abu Ghraib prison and could undermine arguments that they were simply following orders, military legal experts said Thursday.

Sivits, the first of seven U.S. soldiers charged with prisoner abuse to be convicted, told an Army judge that he knew what he was doing was wrong, saying, "Sir, I am truly sorry. I am sorry for what I've done."

The 24-year-old Army reservist agreed to testify against his fellow soldiers in the 372nd Military Police Company in exchange for a lighter sentence. A judge ordered him to spend a year in jail for failing to stop the abuse and for taking a photograph of prisoners being tormented.

The sentence was the maximum amount allowed in a special court-martial, the type of proceeding called in Sivits's case. Three other soldiers charged with abuse face more serious general courts-martial and much longer prison terms if they are found guilty of beating and humiliating prisoners.

Sivits "made it appear he was genuinely sorry," said Stephen A. Saltzburg, a law professor at George Washington University and a former Air Force lawyer. "It gave him a fairly substantial amount of credibility."

In his testimony, Sivits placed all but one of the other charged soldiers at a cellblock in the prison the night of Nov. 8, when he said they variously punched, stomped and kicked seven detainees, stripped them naked and made them simulate sexual acts while being photographed.

Three of them, Spc. Charles A. Graner Jr., Staff Sgt. Ivan L. "Chip" Frederick II and Sgt. Javal S. Davis, were arraigned Wednesday and are scheduled to return to court on June 21 for pretrial hearings. The soldiers will enter their pleas at that point and make any arguments for dismissing charges or changing the venue for the trials.

Sivits's testimony "is going to make some of them think seriously about pleading" guilty, Saltzburg said.

David Sheldon, a Washington civilian who acts as a defense attorney in military courts, said Sivits helped the Army prosecution in another way. By identifying himself as one of the photographers of the hundreds of images taken of detainees and their alleged abusers, Sivits has laid a foundation for allowing the photos to be admitted as evidence, Sheldon said.

He said the maximum jail term Sivits received also could be a signal that the judge, Col. James Pohl, will not go easy on the other soldiers if they are found guilty. "The message you are getting from the judge maxing Sivits out is that the punishment is going to be extremely harsh," Sheldon said.

Sivits did not assert that his commanders or military intelligence officers had encouraged the abuse. But his defense attorney, 1st Lt. Stanley L. Martin, raised this potential line of defense in his closing argument and asked Pohl to take into account an investigative report by Maj. Gen. Antonio M. Taguba that faults Army, CIA and civilian contractors working as interrogators for creating an atmosphere at the prison that encouraged abuse.

Martin asked Pohl to enter the lengthy report into evidence. Pohl thumbed through it on the wooden bench in the makeshift courtroom at the Baghdad Convention Center and asked Martin if he wanted him to read the whole thing. After Martin hesitated, Pohl told him he would read it before making a decision, noting, "We have all the time in the world."

He tucked the report under his arm when he closed the court to consider Sivits's sentence. Twenty minutes later he emerged and imposed the maximum penalty.

Defense attorneys for other defendants "cannot significantly rely on the Taguba report," Sheldon said. "They are going to have to step up and show [that] people in the chain of command were responsible" in as much detail as possible. "It's not going to be enough for the defense to be that military intelligence told them to do it. Making bold assertions that the secretary of defense is responsible doesn't do it."

Elizabeth Lutes Hillman, a professor at Rutgers University law school in New Jersey, said that because Sivits's involvement in the abuse was so brief -- about 30 minutes one night in November out of the two months that the mistreatment is alleged to have occurred -- his case would not help defense attorneys or prosecutors finger higher-ups.

"He seemed to stumble on this, frankly," she said of Sivits. "He will be less help to get information to prosecute those up the chain of the command."