By Paul Von Zielbauer

Last December, when the Marine Corps charged four infantrymen with killing Iraqi civilians in Haditha, Iraq, in 2005, the allegation was as dark as it was devastating: after a roadside bomb had killed their buddy, a group of marines rampaged through nearby homes, massacring 24 innocent people. In Iraq and in the United States, the killings were viewed as cold-blooded vengeance. After a perfunctory military investigation, Haditha was brushed aside, but once the details were disclosed, the killings became an ugly symbol of a difficult, demoralizing war. After a fuller investigation, the Marines promised to punish the guilty. But now, the prosecutions have faltered. Since May, charges against two infantrymen and a Marine officer have been dismissed, and dismissal has been recommended for murder charges against a third infantryman. Prosecutors were not able to prove even that the killings violated the American military code of justice.

Now their final attempt to get a murder conviction is set to begin, with a military court hearing on Thursday for Staff Sgt. Frank D. Wuterich, the last marine still facing that charge. He is accused of killing 18 Iraqis, including several women and children, after the attack on his convoy. If the legal problems that have thwarted the prosecutors in other cases are repeated this time, there is a possibility that no marine will be convicted for what happened in Haditha. Nor is it yet clear whether officers higher up the chain of command than Sergeant Wuterich will be held responsible for the inadequate initial investigation. At least one of the four Marine officers charged last December for failing to investigate the civilian deaths appears to be headed to court-martial. That officer, Lt. Col. Jeffrey R. Chessani, commander of Third Battalion, First Marines, "did not take personal action to fully investigate the actions leading to civilian deaths," concluded Col. Christopher C. Conlin, the officer who examined the evidence. But the case against Capt. Randy W. Stone, the battalion lawyer charged with failing to find out why so many civilians had been killed, was thrown out by Lt. Gen. James N. Mattis, whose decisions in the Haditha prosecutions are final. Charges against First Lt. Andrew A. Grayson, an intelligence officer, are in limbo because of his argument that the Marine Corps has discharged him.

In a wide range of cases involving abuses by American troops in Iraq and Afghanistan, prosecutions have tended to focus on enlisted men and noncommissioned officers — those accused of having personally committed the acts — not on officers who commanded the units. And while there have been numerous convictions, there have also been many cases in which plea arrangements allowed for lesser punishments, or in which charges were dropped or found not to be warranted. The sole officer to face criminal charges in the abuses of prisoners at Abu Ghraib, Iraq, was convicted Tuesday on only one minor charge and will be reprimanded, Reuters reported, quoting an Army announcement. The officer, Lt. Col. Steven L. Jordan, faced five years in prison and dismissal from the Army, but a court-martial decided on the milder penalty, the Army said. The court-martial acquitted him of the charge of being responsible for cruel treatment of detainees at Abu Ghraib.

Experts on military law said the difficulty in prosecuting the marines for murder is understandable, given that action taken in combat is often given immunity under the Uniform Code of Military Justice. "One could view this as a case crumbling around the prosecutor's feet, or one could see this as the unique U.C.M.J. system of justice in operation," said Gary D. Solis, a former Marine judge who teaches the laws of war at Georgetown University Law Center and at West Point. Prosecuting the Haditha case was especially difficult because the killings were not thoroughly investigated when they first occurred. Months later, when the details came to light, there were no bodies to examine, no Iraqi witnesses to testify, no damning forensic evidence. On the other hand, some scholars said the spate of dismissals has left them wondering what to think of the young enlisted marines who, illegally or not, clearly killed unarmed people in a combat zone. "It certainly erodes that sense that what they did was wrong," Elizabeth L. Hillman, a legal historian who teaches military law at Rutgers University School of Law at Camden, said of the outcomes so far. "When the story broke, it seemed like we understood what happened; there didn't seem to be much doubt. But we didn't know." Walter B. Huffman, a former Army judge advocate general, said it was not uncommon in military criminal proceedings to see charges against troops involved in a single episode to fall away under closer examination of evidence, winnowing culpability to just one or two defendants. When Sergeant Wuterich, the soft-spoken squad leader who faces the most extensive murder charges in the Haditha matter, walks into court here on Thursday, "all the prosecutorial attention is now going to center on him," Mr. Solis said. Sergeant Wuterich's lawyers have an uphill legal fight. First, unlike the other marines who faced murder charges, Sergeant Wuterich is charged with the close-range killing of five unarmed men who were ordered out of a vehicle that rolled up near the scene. Also, as a noncommissioned officer and the ranking member of the squad, Sergeant Wuterich may be used by prosecutors to argue that he had a greater responsibility to discern proper targets and avoid civilian casualties. He also led the attack against or was present in every house where civilians were killed.

But the earlier cases show that the defense has some opportunities, too. The presiding officer, Lt. Col. Paul J. Ware, is the same Marine lawyer who conducted hearings for Justin L. Sharratt and Stephen B. Tatum, two other lance corporals accused of killing a total of five Iraqis in three homes in Haditha. Colonel Ware later recommended dismissing the charges against those two men, and he has said the killings should be viewed in the context of combat against an enemy that ruthlessly employs civilians as cover. He warned that murder charges against marines could harm the morale of troops still in Iraq. General Mattis's statements expressing sympathy for the plight of other enlisted marines whom he cleared of wrongdoing in Haditha may indicate his willingness to see Sergeant Wuterich's case in a similar light.

Regardless of what happened to charges against the other defendants, there is still great public pressure on the Marine Corps to investigate and punish any wrongdoing in a case in which so many civilians died. "We can't say those guys didn't commit a crime," said Michael F. Noone Jr., a retired Air Force lawyer and law professor at Catholic University of America. "We can only say that after an investigation, there was not sufficient evidence to prosecute."