BAGHDAD, Iraq, AP -The U.S. military is bracing for a major scandal over the alleged slaying of Iraqi civilians by Marines in Haditha — charges so serious they could threaten President Bush’s effort to rally support at home for an increasingly unpopular war.

And while the case has attracted little attention so far in Iraq, it still could enflame hostility to the U.S. presence just as Iraq’s new government is getting established, and complicate efforts by moderate Sunni Arab leaders to reach out to their community — the bedrock of the insurgency.

U.S. lawmakers have been told the criminal investigation will be finished in about 30 days. But a Pentagon official said investigators believe Marines committed unprovoked murder in the deaths of about two dozen people at Haditha in November.

With a political storm brewing, the top U.S. Marine, Gen. Michael W. Hagee, is headed to Iraq to personally deliver the message that troops should use deadly force “only when justified, proportional and, most importantly, lawful.”

Haditha is not the only case pending: On Wednesday, the military announced an investigation into allegations that Marines killed a civilian April 26 near Fallujah. The statement gave no further details except that “several service members” had been sent back to the United States “pending the results of the criminal investigation.”

Last July, Iraq’s ambassador to the United Nations, Samir al-Sumaidaie, accused the Marines of killing his 21-year-old cousin in cold blood during a search of his family’s home in Haditha, a city of about 90,000 people along the Euphrates River 140 miles northwest of Baghdad.

The military ordered a criminal investigation but the results have not been announced.

Together, the cases present the most serious challenge to U.S. handling of the Iraq war since the Abu Ghraib prison scandal, which Bush cited Thursday as “the biggest mistake that’s happened so far, at least from our country’s involvement in Iraq.”

“What happened at Haditha appears to be outright murder,” said Marc Garlasco of Human Rights Watch. “It has the potential to blow up in the U.S. military’s face.”

He said that “the Haditha massacre will go down as Iraq’s My Lai,” a reference to the Vietnam War incident in which American soldiers slaughtered up to 500 civilians in 1968.

The Haditha case involves both the alleged killing of civilians and a purported cover-up of the events that unfolded Nov. 19.

That day, Lance Cpl. Miguel Terrazas, 20, of El Paso, Texas, was killed by a roadside bomb in Haditha, a Sunni Arab city considered among the most hostile areas of Iraq.

After the blast, insurgents attacked a joint U.S.-Iraqi patrol with small-arms fire, triggering a gunbattle that left eight insurgents and 15 Iraqi civilians dead, the Marines said in a statement issued the following day.

That version stood for four months until a videotape shot by an Iraqi journalism student surfaced, obtained by Time magazine and then by Arab television stations. The tape showed the bodies of women and children, some in their nightclothes.

Although the tape did not prove Marines were responsible, the military began an investigation. Residents came forward with claims that Marines entered two homes and killed 15 people, including a 3-year-old girl and a 76-year-old man — more than four hours after the roadside bombing.

It isn’t clear if questions have been raised about the eight slain people that the Marines described as insurgents.

In March, Lt. Gen. Peter Chiarelli, the No. 2 U.S. commander in Iraq, said about a dozen Marines were under investigation for possible war crimes in the incident. Three officers from the unit involved have been relieved of their posts.

Such incidents have reinforced the perception among many Iraqis who believe American troops are trigger-happy — a characterization U.S. officers strongly dispute.

“America in the view of many Iraqis has no credibility. We do not believe what they say is correct,” said Sheik Sattar al-Aasaf, a tribal leader in Anbar province, which includes Haditha. “U.S. troops are a very well-trained and when they shoot, it isn’t random but due to an order to kill Iraqis. People say they are the killers.”

Ayda Aasran, a deputy human rights minister, said Iraqis should be allowed to investigate such cases — something the U.S. command has refused to permit.

Sunni political leaders will find it difficult to defend U.S. actions, even those aimed at establishing the truth, if they want to maintain their position as leaders of the Iraqi minority that provides most of the insurgents.

Even if criminal charges are brought in the Haditha incident, Sunni insurgents are likely to claim the case is simply a charade and argue that the Marines will escape serious punishment.

Haditha, site of a major hydroelectric dam, has long been considered a tough case. It is among a string of Euphrates Valley towns used by insurgents and foreign fighters to infiltrate from Syria to reach Baghdad and the Sunni heartland.

Many Marines have complained to journalists that they conduct repeated sweeps through villages to drive out the insurgents, who then reappear when the Americans leave. That has bred a sense of frustration among troops fighting a difficult war with no end in sight.

Reporters who embedded in Haditha several months before the alleged massacre said Marines considered the town as enemy territory, with frequent roadside bombings. During patrols inside the city, Marines treated inhabitants like terrorists, raiding their homes.

An Associated Press journalist who traveled in Haditha last June with a Marine unit not involved in the November killings saw a Marine urinate on the kitchen floor of a home and on another occasion saw insults chalked in English on the gate of an Arab home. The reporter asked a Marine commander about the incident and was told it would be investigated.

Last August, the British newspaper The Guardian reported that Haditha was under the control of religious extremists who enforced their own strict interpretation of Islamic law — including decapitations of people suspected of collaborating with the Americans.

“This is a war in which the distinction between killing the enemy and massacring civilians is not always completely obvious,” said John Pike of GlobalSecurity.org. “Counterinsurgency operations are particularly prone to the killing of people who, in retrospect, are judged to have been innocent civilians, but who in the heat of battle seemed to be the enemy.”

Some analysts, however, say the killings of civilians also reflect frustration among young troops fighting a difficult war with no end in sight. They say these young fighters have been thrust into an alien culture for repeated tours in a war whose strategy many of them do not understand.

“What we’re seeing more of now, and these incidents will increase monthly, is the end result of fuzzy, imprecise national direction combined with situational ethics at the highest levels of this government,” said retired Air Force Col. Mike Turner, a former planner at the Joint Chiefs of Staff.