SAN DIEGO — A Camp Pendleton Marine sergeant was ordered today to stand trial on charges of unpremeditated murder and dereliction of duty in the killing of an unarmed detainee in Fallujah, Iraq.

Lieutenant General Samuel Helland ordered the court-martial of Sergeant Ryan Weemer after finding there was sufficient evidence to send him to trial.

Sergeant Weemer is one of three current and former Marines accused of breaking rules of engagement and killing four men they had captured after a platoon commander radioed to ask whether the Iraqis were “dead yet.”

A telephone message left by for Sergeant Weemer’s attorney, Paul Hackett, was not immediately returned.

The killings occurred in November 2004 during the invasion of Fallujah, one of the fiercest ground battles of the Iraq war.

The case came to light in 2006, when Sergeant Weemer volunteered details to a U.S. Secret Service job interviewer during a polygraph screening that included a question about the most serious crime he had ever committed.

Sergeant Weemer, of Hindsboro, Ill., is charged with one count of murder and six counts of dereliction of duty encompassing failure to follow the rules of engagement in Fallujah and failing to follow standard operating procedures for apprehending or treating detainees or civilian prisoners of war.

General Helland’s decision to order the court-martial follows an Article 32 hearing, similar to an evidentiary hearing, where prosecutors argued that Sergeant Weemer, a burly 25-year-old honored with a Purple Heart, should be tried for unpremeditated murder because he knew the rules of engagement forbade harming anyone in his custody.

During the hearing last month, prosecutors played a tape recording of the Secret Service interview where Sergeant Weemer recounted arguing with his squadmates about what to do with the detainees — all military-age males captured in a house where weapons were also found. The squad was under pressure from the platoon to get moving.

A Marine Corps spokesman, Lieutenant Colonel David Griesmer, said Sergeant Weemer next faces arraignment on the charges at Camp Pendleton. A date has not been set.

The commanding general of the 1st Marine Expeditionary Force and Marine Corps Central Command, General Helland, followed the recommendations of the investigating officer, Major Glen Hines, to order the court-martial.

Sergeant Weemer’s attorney has put much of the blame on Sergeant Weemer’s former squad leader, saying Jose Nazario Jr. escalated the situation inside the house by beating one detainee with the butt of a rifle after the weapons cache was found.

Mr. Nazario, 27, of Riverside, Calif., was charged with two counts of voluntary manslaughter in the killings of two captives, but a spokesman for federal prosecutors said today that the charges changed with a superceding indictment handed down in June.

The former sergeant is now charged with one count of voluntary manslaughter for allegedly killing or causing others to kill four unarmed detainees, along with one count of assault with a deadly weapon and one count of discharging a firearm during a crime of violence. The charges were filed in federal court because he has already completed his military service.

Mr. Nazario has pleaded not guilty; his trial is set to begin Aug. 19.

Another Marine, Sergeant Jermaine Nelson, 26, of New York is slated to be court-martialed in December on charges of unpremeditated murder and dereliction of duty for his role in the alleged killings.

Although he has not entered a formal plea in military court, Sergeant Nelson’s attorney has repeatedly said his client was innocent.

Sergeants Nelson and Weemer had both been jailed in June for contempt of court for refusing to testify against Mr. Nazario before a federal grand jury believed to be investigating the case. Both were released July 3 and returned to Camp Pendleton.