The travelers entered Fallujah first through a checkpoint operated by the Iraqi Civil Defense Corps, a U.S.-trained paramilitary unit meant to add muscle to the American-led occupation. The men in black berets distractedly waved cars past, onto the city's main street.

Then it became apparent who was really in charge. A few yards in, wild-eyed young men in masks pulled cars over at will, searched them and demanded identification documents. No one could leave or enter without passing muster. Other groups of fighters in masks roamed side streets and alleys, brandishing rifles at all sorts of angles.

It was not supposed to be like this. Under an agreement made last month with U.S. Marine commanders, a new force called the Fallujah Brigade, led by former officers from Saddam Hussein's demobilized army, was to safeguard the city. The unruly gunmen -- many of them insurgents who battled the Marines through most of April -- were supposed to give way to Iraqi police and civil defense units.

Instead, the brigade stays outside of town in tents, the police cower in their patrol cars and the civil defense force nominally occupies checkpoints on the city's fringes but exerts no influence over the masked insurgents who operate only a few yards away.

The Marines gave the brigade the task of apprehending the killers of four American contractors whose bodies were burned, mutilated and hung from a bridge in March, capturing foreign fighters and disarming the insurgents. None of that has happened.

President Bush endorsed the Fallujah solution on the grounds that it made "security a shared responsibility." But the sight of insurgents still in control of byways and the kidnapping of foreigners and Iraqis with impunity suggests that they are sharing their power with no one.

Moreover, continuing mayhem on Fallujah's outskirts raises the question of whether the Americans have simply created a safe haven for anti-occupation fighters. On Saturday, a Fallujah-based group calling itself the Mujaheddin Battalions announced it was transferring its fight to Baghdad -- but was still committed to the truce in its home city.

Fallujah byways are a hell of roadside bombs and ambushes. On Friday, an armored sport-utility vehicle carrying this Washington Post reporter and his driver was attacked close to Fallujah on the main highway to Baghdad. Four men in an orange-and-white taxi pumped dozens of bullets from AK-47 assault rifles into the vehicle for more than two minutes, each round causing a loud thump on the vehicle's metal plating and reinforced windows. They shot from behind, from in front and from the sides, where their determined frowns and mustached faces were clearly visible, as they and we weaved down the highway at 90 mph. The fusillade stopped when the SUV, its back tires missing and its rear windows shattered, spun out of control. The gunmen sped down the road, evidently thinking their mission was accomplished. Neither the driver nor the reporter was injured.

Marines were once determined to put an end to the threats in and around Fallujah. After the April fighting, they were poised to stage a full-blown assault on Fallujah, but a public outcry over casualties and calls from Iraqi allies for a negotiated solution led to the new arrangement. A similar dynamic has slowed the U.S. pursuit of Shiite Muslim rebels in southern Iraq, where fear of igniting a broader revolt has stayed the hand of U.S. forces. So far, two cease-fires have been called in the south.

In Fallujah, despite the compromise, the Marines' brash adversaries dominate the streets. Yet no U.S. offensive is in the works.

"We don't intend to go in wholesale. There's no doubt we could clear Fallujah out, but to what end?" asked Col. Larry Brown, an operations officer with the 1st Marine Expeditionary Force camped outside the city. "We measure progress in small steps. We prefer to bring them back into the fold slowly. It is a good sign that Iraqis are handling their problems."

Brown acknowledged that Marines were concerned that Fallujah could become a guerrilla staging area, but said, "it is only supposition that Fallujah is a sanctuary for insurgents. If it is in any way, then the deal's off. There is probably a small contingent of hard-core gunmen there," he said.

"Inevitably, if we went in, there would be a lot of collateral damage. People would defend their homes. We would only go as a last resort," Brown said.

Fallujah, about 35 miles west of Baghdad in an area known as the Sunni Triangle, has been a center of anti-occupation rebellion among Sunni Muslims for more than a year. It is a city renowned for smugglers and for supplying recruits for Hussein's army and security services. It is also known for piety; residents call Fallujah "the city of 100 mosques," several of which were used as redoubts for fighters firing on Marines.

Since the truce, Islam has emerged openly as a potent force, according to Brown and Iraqis familiar with the city. Islamic law, or sharia, is beginning to take root, to the point where clandestine vendors of alcohol have been flogged and paraded naked on the street; beauty salons have been shut down and barbers told to eschew Western cuts and not shave off beards. Among strict Muslims, beards are a requirement.

Foreigners have frequently been kidnapped by gangs of masked gunmen, who have released their captives only in response to requests by religious leaders.

On Friday, masked men in Fallujah handed out a manifesto signed by 18 groups with names such as the God Is Great Battalions, the Muhammad Messenger of God Forces, the Islamic Resistance Brigades and the Jihad Battalion. They rejected Iraq's newly named interim government and accused the United States of "acts of killing, destruction, violation of holy places and organized plunder."

"America shall reap, with God's help, what it has sown," the document said. It pledged to "continue resistance in all its forms as the only way to achieve victory."

Fallujah residents who supported the creation of the U.S.-backed Fallujah Brigade said the city was unsettled but not out of control. Jasim Saleh, one of the brigade's commanders, said "it is a mystery" who was kidnapping foreigners and that he opposed imposition of Islamic law in the city. Insurgents who came to Fallujah from elsewhere are being pressured by local leaders to leave, he said, adding that the killers of the American contractors have probably already fled the city. In any case, he said, "Everyone in Fallujah condemns the mutilation."

Saleh said there was no need for the Marines to resume patrolling the city, because about 1,700 brigade members are equipped to take control. "There are still influences in Fallujah trying to spoil the accord. Many are from outside the city and some from outside Iraq, but we will soon be able to dominate Fallujah," the former army officer said in a telephone interview.

Esawi Barakat, a local tribal leader described by Marines as a power broker in the city, played down the imposition of Islamic law. Restrictions on alcohol and hairdressers only complement Fallujah's conservative personality, he argued.

"After so many deaths, people who misbehave in front of Fallujah's families must be punished," he said. "If you want to drink, you should go to Baghdad. Western hairstyles offend our rural tastes."

Barakat said that danger to foreigners originates with a lust for revenge after a year of turmoil that climaxed with heavy fighting in April and May. "There have been too many dead. People only think of revenge," he said. "If people discover a visitor is an American agent or traitor, they are all happy to punish him."

U.S. forces jailed Barakat for six months on suspicion of leading anti-occupation actions. He was released in April. If the city is tense, he argued, it is because residents suspect the Marines will move in again. Another tribal leader and former army general, Khalaf Aliyan Khalaf, said that keeping Marines out of Fallujah is only the first step.

"As long as the Americans are inside Iraq, there will be no security anywhere," he said.

Saleh, Barakat and Khalaf all said that forces such as the Fallujah Brigade ought to be given control of other cities in central Iraq. Brown said it was too early to expand the experiment. "The jury is still out on Fallujah," the Marine colonel said.

In Fallujah on Friday, neither police nor the brigade showed any eagerness to clear the streets of masked men. Their inaction made for a nerve-wracking trip. When my driver and I approached four policemen who were sitting in a squad car and asked them to escort us out of the city, one answered, "If we did that, they would kill us as spies, and kill you, too."

The policemen suggested that the best escape route lay on the northeast side of town, on the approach to Fallujah Brigade headquarters. As it turned out, the insurgents had set up another checkpoint there, but the gunmen manning the post remained huddled in thin shade and did not pay heed to The Washington Post's vehicle as it passed.

At the brigade headquarters, a group of recruits stood idly among new U.S.-installed tents in a small military complex. Brigade members said that they had not entered Fallujah for several days but insisted that the masked men had no authority to stop anyone. "We are all cooperating, so it does not make any difference if we are there or not," said one guard.

The brigade has been billed as a trained unit of former Iraqi soldiers, with some additions of Fallujah fighters. At their base, a group of brigade members appeared to be unimpressed by their chain of command. They repeatedly interrupted a portly man, who said he was the commander on duty, when he advised us to move north, away from the city. The men insisted instead on escorting us back into Fallujah, and from there, would lead us to the highway. They said there was no way to get on the road to Baghdad by heading north. "Come with us. We will protect you, no problem," a bearded man said.

The suggestion appeared odd, since the Fallujah police had said there was a gravel on-ramp to the highway just a few miles north of the brigade camp. We turned down the offer.

Instead, we decided to follow a U.S. military convoy just a few hundred yards away. The convoy had stopped because someone had spotted a roadside bomb, and the troops were waiting for engineers to arrive and blow it up. The Fallujah Brigade members then tried to block the Post vehicle from proceeding when the U.S. troop convoy moved out. They allowed us to pass only when a U.S. military Humvee topped by a menacing machine gun rolled back to the brigade headquarters to see what was going on.

On the highway, the military convoy peeled off to travel to its home base just east of Fallujah. The Washington Post vehicle continued toward Baghdad. Ten miles down the road, the orange-and-white taxi carrying the gunmen appeared and began firing.

Despite damage to the vehicle, it eventually limped to Abu Ghraib prison, about 20 miles west of Baghdad, where U.S. military police gave us refuge. Few residents of the notorious facility probably ever entered the compound as happily as we did.

Iraqi Civil Defense Corps officers secure an area along the Baghdad-Fallujah highway last week after a U.S. Army vehicle was attacked by Iraqi militiamen.