The biggest U.S. criminal case involving civilian deaths in the Iraq war entered a military courtroom for the first time Tuesday with a hearing for a Marine officer charged with failing to properly investigate the killings of 24 Iraqis in Haditha.

Capt. Randy W. Stone is one of four officers accused of failing to report and investigate the Nov. 19, 2005, killings. All four are charged with dereliction of duty.

Three enlisted Marines are charged with unpremeditated murder in the case. They deny any wrongdoing, saying they responded properly to a perceived threat during a highly chaotic time.

The hearing for Stone, a military attorney from Dunkirk, Md., will review whether he should have launched an investigation sooner. An investigating officer at the Article 32 hearing, the military's equivalent of a grand jury proceeding, will recommend whether the charges should go to trial.

Stone, 34, appeared wearing desert camouflage, flanked by his civilian attorney and two military defense lawyers.

Stone's civilian attorney, Charles Gittins, has said he expects to call several witnesses to testify, including a two-star general and a sergeant who was involved in the killings.

The slayings occurred after a roadside bomb destroyed a Humvee in a Marine convoy, killing Lance Cpl. Miguel Terrazas of El Paso, Texas, and injuring two of his comrades. In the aftermath, Marines shot five Iraqi men as they approached the scene in a car and others — including women and children — died as troops went house to house in the area, clearing homes with grenades and gunfire.

Differences emerge on whether the enlisted Marines' actions should be called murder — and whether their commanders failed in their duty to investigate.

Initially, the Marines were praised for their actions in Haditha. Months later, Time magazine published a story that questioned the official version, prompting an investigation.

A report by Army Maj. Gen. Eldon A. Bargewell said Marine officers did not deliberately cover up the incident, but it faulted the Marine chain of command for viewing civilian casualties, "even in significant numbers, as routine."

The squad leader, Staff Sgt. Frank D. Wuterich, is charged with the unpremeditated murder of 18 of the Iraqis.

Gittins has said his client did nothing wrong because he reported the facts to his superiors as he understood them.

"They are saying that he should have nipped at people's heels telling them they need to investigate," Gittins said last week. "Everything that my client knew, they knew."

Former military prosecutor Tom Umberg said just because superior officers do not take action "that doesn't absolve you of an obligation" to report a suspected law of war violation to military investigators.

Prosecutors have given at least eight other Marines immunity in return for their testimony. One, Sgt. Sanick P. Dela Cruz, initially was charged with the unpremeditated murder of five civilians.