No American serviceman has been executed since 1961. But in the past month, new cases in Iraq have led to charges against 12 American servicemen who may face the death penalty in connection with the killing of Iraqi civilians.

Military officials caution against seeing the cases as part of any broader pattern, noting that the incidents in question are isolated and rare. But the new charges represent an extraordinary flurry in a conflict that has had relatively few serious criminal cases so far.

As investigators complete their work, military officials say, the total of American servicemen charged with capital crimes in the new cases could grow substantially, perhaps exceeding the total of at least 16 other marines and soldiers charged with murdering Iraqis throughout the first three years of the war.

Some military officials and experts say the new crop of cases appears to arise from a confluence of two factors: an increasingly chaotic and violent war with no clear end in sight, and a newly vigilant attitude among American commanders about civilian deaths.

At least five separate incidents involving the deaths of Iraqis are under investigation, setting off the greatest outcry against American military actions since the Abu Ghraib prison scandal. By far the best known of the cases is the one in Haditha, where marines are being investigated in the killings of 24 Iraqi civilians in November. No charges have been filed in that case, but some say news of the incident may have helped bring some later cases to light.

"Unusual criminal acts raise the level of concern, whether in the military or among civilians, and with increased concern comes increased reporting," said Gary Solis, a former marine who teaches the law of war at Georgetown University.

In April, Lt. Gen. Peter W. Chiarelli, the No. 2 American commander in Iraq, issued an order that specified for the first time that American forces must investigate any use of force against Iraqis that resulted in death, injury or property damage greater than $10,000. Maj. Todd Breasseale, a spokesman for the American command, said he knew of no clear link between General Chiarelli's order and the recent homicide investigations.

But Major Breasseale said that General Chiarelli, who took over day-to-day military operations in Iraq in January, has made clear to subordinates that he puts a high priority on avoiding and scrupulously reporting civilian casualties. American commanders in Iraq will be scrutinizing civilian deaths more intensely as the United States moves toward transferring authority to Iraqis, Major Breasseale said. Details about the five incidents under investigation are still emerging, and none of those charged have yet had an Article 32 hearing, the military's equivalent of a grand jury proceeding.

The incidents are far from the only ones in which American forces killed Iraqis. But serious criminal charges in such cases have been rare until now. In many earlier cases, the killings have been found to be justifiable, and the soldiers or marines in question have often been handled through administrative or nonjudicial processes.

The last soldier to be executed was John A. Bennett, hanged in 1961 after being convicted of the rape and attempted murder of an 11-year-old Austrian girl.

In the Iraq war, when soldiers or marines have been charged, convictions — and harsh sentences — have been rare. Of the 16 American servicemen known to have been previously charged with murder, only six were convicted or pleaded guilty to that charge, and none received the death penalty. In all, 14 service members have been convicted of any charge in connection with the deaths of Iraqis and have received sentences as varied as life in prison or dismissal from the service.

Among the new incidents, all five took place in central Iraq, in areas where the Sunni Arab insurgency is firmly entrenched despite years of effort to quell it by American and Iraqi forces. To some, that is the only thing that seems to link them.

"This is a war in which soldiers and civilians are constantly mingling, and they often don't understand each other," said Loren B. Thompson, a military analyst at the Lexington Institute. "The enemy has a conscious strategy of demoralizing U.S. forces by disorienting and confusing them. Against that backdrop, the number of alleged atrocities is quite low compared with other conflicts in the past."

In Vietnam, a much longer conflict, 95 American soldiers and 27 marines were convicted of killing noncombatants.

Some of the men under investigation in Iraq had done multiple tours in Iraq, and that, too, may have played a role.

"They can become almost numb to the killing," said Charles W. Gittins, a former marine and a lawyer who has represented marines accused of murder in Iraq. "The more you're in it, the more you want to live through it. You think more about preserving your own life than about what's the right thing to do."

In many of the cases where American troops killed Iraqi civilians, they were later found to have acted within their rules of engagement. Some of those cases became notorious, at least in the Arab world.

In Falluja in November 2004, for instance, the freelance journalist Kevin Sites filmed a Marine corporal shooting an apparently wounded and unarmed Iraqi in a mosque. The videotape generated a frenzy of negative publicity, but in May 2005 a military review cleared the corporal, stating that he had acted within the rules of engagement.

The definition of murder can be far more elusive in a war zone than in civilian life. In some previous criminal cases, soldiers or marines have claimed they acted in self-defense or carrying out mercy killings.

Cpl. Dustin M. Berg of the Indiana National Guard, who was sentenced to 18 months in prison for killing his Iraqi police partner, said he acted because he feared his partner was going to shoot him.

In 2004, Staff Sgt. Johnny Horne said he killed a wounded 16-year-old Iraqi boy to put him out of his misery after a gun battle with Shiite militants.

Sergeant Horne pleaded guilty to unpremeditated murder and was sentenced to three years in prison, later reduced to one year.

In the heaviest penalty yet issued in the Iraq war, Sgt. Michael P. Williams was sentenced to life in prison after being convicted of premeditated murder last year in the killing of two Iraqi civilians in Baghdad. The sentence was later reduced to 25 years.

"I think there's a recognition that these are weird environments," said Eugene R. Fidell, a specialist in military law. "The danger is, carried to an extreme, that can mean throwing the law books out."

The flurry of new cases has taken on a high profile in the news media and public discussion. The barriers to conviction, though, will be formidable. Recovering credible evidence in Iraq's chaos can be very difficult, and Iraqi witnesses are open to challenge.

"There's going to be very little forensic evidence," Mr. Gittins said. "Jury members who have served in Iraq know that it is pretty common for Iraqis to lie to Americans. Also, the military pays the relatives of civilians who are killed — so they have an incentive to lie."

Some members of the military juries are likely to have served in Iraq, and are familiar with the chaotic atmosphere surrounding any decision to use force. "The presumption of innocence is going to reign supreme," Mr. Gittins said.