10 year old boy grinds his heel into a burned head: "Where is Bush?" the little boy yelled. "Let him come here and see this!" The US Army will hunt this boy down and fire a bullet into his head, I hope.

FALLUJA, Iraq, March 31 — Four Americans working for a security company were ambushed and killed Wednesday, and an enraged mob then jubilantly dragged the burned bodies through the streets of downtown Falluja, hanging at least two corpses from a bridge over the Euphrates River.

Less than 15 miles away, in the same area of the increasingly violent Sunni Triangle, five American soldiers were killed when a roadside bomb ripped through their armored personnel carrier.

The violence was one of the most brutal outbursts of anti-American rage since the war in Iraq began more than a year ago. And the steadily deteriorating situation in the Falluja area, a center of anti-American hostility west of Baghdad, has become so precarious that no American or Iraqi forces responded to the attack against the civilians, who worked for a North Carolina company.

American officials said the civilians were traveling in two sport utility vehicles although some witnesses in Falluja said there were four. "Two got away; two got trapped," said Muhammad Furhan, a taxi driver.

It is not clear what the four Americans were doing in Falluja or where they were going. But just as they were passing a strip of stationery stores and kebab shops around 10:30 a.m., masked gunmen jumped into the street and blasted their vehicles with assault rifles. Witnesses said the civilians did not shoot back.

There are a number of police stations in Falluja and a base of more than 4,000 marines nearby, but even as the security guards were being swarmed and their vehicles set on fire, sending plumes of inky smoke over the closed shops of the city, there were no ambulances, no fire engines and no assistance.

Instead, Falluja's streets were thick with men and boys and chaos.

Men with scarves over their faces hurled bricks into the blazing vehicles. A group of boys yanked a smoldering body into the street and ripped it apart. Someone then tied a chunk of flesh to a rock and tossed it over a telephone wire.

"Viva mujahedeen!" shouted Said Khalaf, a taxi driver. "Long live the resistance!"

Nearby, a boy no older than 10 ground his heel into a burned head. "Where is Bush?" the boy yelled. "Let him come here and see this!"

Masked men gathered around him, punching their fists into the air. The streets filled with hundreds of people. "Falluja is the graveyard of Americans!" they chanted.

Several news crews filmed the mayhem. The images of a frenzied crowd mutilating bodies were reminiscent of the scene from Somalia in 1993, when a mob dragged the body of an American soldier through the streets of Mogadishu. That moment shifted public opinion and eventually led to an American pullout.

The White House blamed terrorists and remnants of Saddam Hussein's former government for the attack. "This is a despicable attack," Scott McClellan, the White House spokesman, told reporters, adding that "there are some that are doing everything they can to prevent" a transfer of sovereignty to an Iraqi government on June 30.

American military officials said the violence in Falluja, however chilling, would not scare them away. "The insurgents in Falluja are testing us," said Capt. Chris Logan, a marine. "They're testing our resolve. But it's not like we're going to leave. We just got here."

Captain Logan, who is stationed at a large walled base on the outskirts of the city, said Falluja was becoming "an area of greater concern." Last week, a contingent of marines, who recently took over responsibility for Falluja from the Army, fought gunmen in a battle in which one marine, a television cameraman and several Iraqi civilians were killed.

"This is one of those areas in Iraq that is definitely squirrelly," Captain Logan said.

Many people in Falluja said they believed that they had won an important victory on Wednesday. They insisted that the four security guards, who were driving in unmarked sport utility vehicles, were working for the Central Intelligence Agency.

"This is what these spies deserve," said Salam Aldulayme, a 28-year-old Falluja resident.

Intelligence sources in Washington said the four were not working for the C.I.A. They worked for Blackwater Security Consulting of Moyock, N.C., providing security for food delivery in the Falluja area, according to a statement from the company. The occupation authorities have hired hundreds of private security guards for a range of duties.

Witnesses in Falluja said several of the men had Defense Department badges, though such identification is common for contractors working for the occupation. A senior military officer said the four were retired Special Operations forces — three Navy Seals and one Army Ranger. American officials declined to immediately identify the dead men.

In the last three weeks, more than 10 foreign civilians have been killed in Iraq, though no attack provoked the spasm of brutality that followed this one.

Since the war in Iraq began, Falluja has been a flash point of violence. Of all the places in Iraq, it is where anti-American hatred is the strongest. The area is predominantly Sunni Muslim. Many families remain loyal to the captured dictator, Mr. Hussein, who is also a Sunni Muslim. Over the years, Mr. Hussein cultivated a network of patronage and privilege among the tribes and elders of Falluja. Many became top army officers. Some ran big companies. When Mr. Hussein was ousted last April, the people here lost their jobs, their businesses and their power.

That set off a cycle of killing and responses, a bloody feud between a clannish society and occupiers from thousands of miles away. Last April, American soldiers killed more than 15 civilians at a demonstration in Falluja. In November, an American helicopter was shot down outside the town, killing 16. Townspeople danced on the wreckage.

In February, insurgents mounted a brazen daylight attack against a convoy carrying Gen. John P. Abizaid, the American commander in the Middle East. He escaped unscathed. But two days later, gunmen blasted their way into a Falluja jail, killing at least 15 police officers and freeing dozens of prisoners.

Last week, the First Marine Expeditionary Force formally took control of the city, population 300,000, which sits on a desert shelf about 35 miles west of Baghdad. Marine commanders said they were going to try a different approach from the Army, which had basically pulled back to bases ringing Falluja and left policing up to the locals.

"We're doing work outside the wire," Captain Logan said. "We're running patrols. We're rebuilding things. We're working with Iraqis."

Most of the Sunni Triangle, north and west of Baghdad, has become so unsafe that American forces stick to their bases, their movement usually limited to heavily guarded convoys.

Around 7 a.m. on Wednesday, an Army convoy passing through the town of Habbaniya, west of Falluja, rolled over an I.E.D., or improvised explosive device. The bomb was buried in the road and blew up under an armored personnel carrier, killing five soldiers. Roadside bombs are everyday occurrences in Iraq. But few have claimed as many casualties. "It was a very large I.E.D.," said Brig. Gen. Mark Kimmitt, deputy operations director for the occupation forces.

A few hours later the men from Blackwater Security drove into downtown Falluja. After they were shot, the scene turned grisly. A crowd of more than 300 people flooded into the streets. Men swarmed around the vehicles. Some witnesses said the Americans were still alive when one boy came running up with a jug of gasoline. Soon, both vehicles were fireballs.

"Everybody here is happy with this," Mr. Furhan, the taxi driver, said. "There is no question."

After the fires cooled, a group of boys tore the corpses out of the vehicles. The crowd cheered them on. The boys dragged the blackened bodies to the iron bridge over the Euphrates River, about a mile away. Some people said they saw four bodies hanging over the water, some said only two. At sunset, nurses from a nearby hospital tried to take the bodies away.

Men with guns threatened to kill the nurses. The nurses left. The bodies remained.


Christine Hauser contributed reporting from Baghdad for this article.