FORT MEADE, Md. — A military judge yesterday dismissed two of the most serious charges against the only officer accused of abusing detainees at Iraq’s Abu Ghraib prison after an investigator acknowledged he failed to read the defendant his rights.

Army Lieutenant Colonel Steven Jordan, 51, of Fredericksburg, Va., is the last of 12 Abu Ghraib defendants to be court-martialed. Prosecutors yesterday amended one of the four remaining counts against him, a cruelty and maltreatment charge, by narrowing its scope from three months to a single day.

Colonel Jordan’s trial began with jury selection yesterday afternoon. Members of the five- to 15-member jury all will be higher in rank than Colonel Jordan.

Colonel Jordan, the former director of the prison’s interrogation center, was charged after photographs surfaced showing low-ranking American soldiers assaulting and humiliating naked detainees at Abu Ghraib in late 2003 and early 2004. Colonel Jordan isn’t in any of the pictures, but he is accused of allowing the mistreatment to escalate.

Colonel Jordan has argued that he is a scapegoat who, because he is a reservist, is considered expendable.

In court yesterday morning, prosecutor Lieutenant Colonel John Tracy announced that an investigator, Major General George Fay, had contacted prosecutors Sunday to say that he “misspoke” when he testified during a pretrial hearing that he had advised Colonel Jordan of his rights during an interview in 2004.

In that 2004 interview, Colonel Jordan had told General Fay he never saw detainees being abused and never saw nude detainees.

The judge, Army Colonel Stephen Henley, granted the government’s motion to dismiss two charges that were based on those statements: making a false official statement, punishable by up to five years in prison, and obstruction of justice, punishable by up to three years.

Colonel Jordan still is charged with disobeying General Fay’s order not to discuss the investigation with others, punishable by up to five years in prison. The three other counts refer to the treatment of prisoners. Colonel Jordan is charged with failure to obey a regulation, punishable by up to two years in prison; cruelty and maltreatment of detainees, punishable by up to one year, and dereliction of duty, which carries a maximum prison sentence of six months.

General Fay interviewed many other soldiers during his investigation. In his report, he concluded that Colonel Jordan’s tacit approval of violence during a weapons search on November 24, 2003, “set the stage for the abuses that followed for days afterward.” The search, known as the “roundup,” followed an episode in which a Syrian detainee fired at Colonel Jordan and other soldiers with a handgun he had obtained from Iraqi police officers, according to investigative records.

Colonel Jordan’s defense, led by Captain Samuel Spitzberg, contends that although Colonel Jordan was the titular head of the interrogation center, he spent most of his time trying to improve soldiers’ deplorable living conditions at Abu Ghraib.

The defense argued during an October hearing that interrogation conditions were set by two other officers: Colonel Thomas Pappas, an intelligence brigade commander who was the highest-ranking officer at Abu Ghraib, and Captain Carolyn Wood, leader of a unit within the interrogation center called the Interrogation Command Element.

Neither Colonel Pappas nor Captain Wood has been charged with crimes. Pappas was reprimanded and fined $8,000 for once approving the use of dogs during an interrogation without higher approval.

Eleven enlisted soldiers have been convicted of crimes at Abu Ghraib. The longest prison term was given to a former corporal, Charles Graner Jr., of Uniontown, Pa., who was sentenced in January 2005 to 10 years for assault, battery, conspiracy, maltreatment, indecent acts, and dereliction of duty.