The streets outside the house were practically deserted. Most inhabitants of Amriyah, some 30 miles east of Baghdad, were indoors, praying or napping out of the relentless Friday-afternoon sun. Pro-Saddam slogans on the neighborhood's yellowing concrete walls underscored its bleakness. Inside one of the Soviet-style houses, up a flight of stairs, was a small family apartment where three Iraqi resistance fighters had agreed to be interviewed. They emerged from a back room, armed with AK-47s and grenades, their faces hidden by red-and-white kaffiyehs. Seating themselves on floor mats, they talked about the war against America. Their group, calling itself the Army of Mohammed, has claimed responsibility for the deaths of at least 15 U.S. soldiers since the fall of Saddam Hussein. "We did kill U.S. soldiers and we destroyed some of their vehicles and equipment," said the leader of the three, calling himself Mohammed al-Rawi. "We will do it again."

Such threats worry Bush officials more than they want to admit. "We've made good progress," the president said last week, marking the 100th day since he declared an end to major combat. "Iraq is more secure." Nevertheless, 56 Americans were killed in action during those 100 days, an additional 404 Coalition forces were wounded badly enough to be knocked out of duty, and there's no sign that the attacks are letting up. On the contrary, the resistance seems to be getting bigger, smarter and more organized. U.S. officials in Baghdad have estimated its total strength in the thousands, and recently acknowledged that its efforts may be coordinated at the regional level, if not nationally. Coalition officials can't travel without heavy escorts, reconstruction efforts have been hobbled and ambushes have forced even the Red Cross to cut back its operations. Last week a truck bomb killed 19 people at the Jordanian Embassy in one of the bloodiest attacks yet. It wasn't immediately clear who did it or why.

The Army of Mohammed is one of several clandestine groups that sprang up after the regime's collapse. At first it was known only from leaflets found scattered outside Baghdad's Abu Hanifa mosque, where Saddam was last seen in public on April 9. Recently the name has begun appearing on walls in Saddam's hometown, Tikrit. Members of the Army of Mohammed have claimed several attacks on Coalition forces; if those claims are legitimate, the group is one of the deadliest in Iraq. The Americans, though, aren't sure. The group's aims, its acts and its membership are topics of fierce debate among intelligence analysts. Some have even contended that no such armed group really exists.

In order to learn more about the anti-American resistance's motives and methods, NEWSWEEK asked a well-connected intermediary for help in contacting some fighters. He arranged a meeting with al-Rawi, 40, and two comrades, Ali Saadi, 32, and Kadim Baghdadi, 34. The three were carrying illegal weapons, which would get them arrested or shot on sight by U.S. forces. They also seemed well organized, arranging the rendezvous at a location they chose, and arriving and departing precisely on time. They say they have 5,000 armed fighters, and a "centralized" command structure extending west to Ar Ramadi, north to Tikrit and east to Baghdad. There's no way to confirm such details, and the three guerrillas refused to provide specific information on attacks they had carried out, claiming a need to protect "operational security."

Baghdadi says the organization initially was a gathering of tribal fighters, many of whom had served previously in the Iraqi armed forces and had been using firearms since childhood. "Most of our youth are trained to carry weapons," he added. The fighters were angry with U.S. forces for the deaths of 13 Iraqis after an anti-occupation protest turned violent in Fallujah. "Through the key figures of the tribes, we contacted each other," said Baghdadi. "We met in small cells at first, far from the cities, in farms, and we started talking. We took the decision that we must liberate the country."

By all accounts, the fighters are taking Mao Zedong's classic advice for guerrillas to move among the people like fish through water. They live in the civilian population, depending on its support and using it for protection. It's a strategy that severely complicates U.S. efforts to wipe them out.

The group's ideology seems to be a blend of ardent nationalism, Sunni Islamic zealotry and anti-Jewish bigotry. Before the invasion, Saddam Hussein used much the same recipe to rally popular support. Some of the resistance group's members might dream of re-creating the old Baathist regime, but the three who met with NEWSWEEK claimed no wish for Saddam's return. "We want to make a new government, without Saddam but in the same style," said one. "We don't want to bring Saddam back." Still, they could be Saddam loyalists trying to broaden their appeal to other Iraqis who have no love for the fallen dictator.

A much bigger consideration is the group's hatred of America. Al-Rawi was carrying a prepared statement, which he read aloud: "The Americans have occupied our land under a false pretext, and without any international authorization. They kill our women and children and old men. They want to bring the Jews to our holy land in order to control Iraq, to achieve the Jewish dream." The document ended with a pledge of vengeance against the Americans. "We promise we will burn their tanks. They will die."

The fighters say they're doing all they can to bring that day faster. They and others associated with the group describe a rapidly evolving support network. Last week three other members of al-Rawi's cell traveled to the southern town of Al Kut to stock up on arms and ammo. They made the 150-mile run in an ice-cream truck emblazoned with the word frezo, according to a former officer in the Iraqi Intelligence Service, the Mukhabarat, who described the trip to NEWSWEEK. (He was an intermediary for the arms dealers.) Like most towns in the Shiite-dominated south, Al Kut has been relatively quiet in recent weeks. Most inhabitants seem at least cooperative if not pro-American. But a weapons bazaar is flourishing at a housing complex just outside town. The Mukhabarat officer says he and his traveling companions were ushered into house after house. At each stop they were offered tea and then were shown a variety of weapons for sale at bargain prices. The sellers were asking as little as $70 for a rocket-propelled-grenade launcher. Hand grenades were going for $1.50 apiece.

The travelers paid for their purchases with crisp, newly minted bundles of Iraqi currency. They bought at least seven plastic boxes of RPG rounds, several tin boxes of ammunition, several used and damaged RPGs for spare parts and a few pistols. They hid everything under the ice cream, turned up the freezer and headed home, the officer says. The driver followed a circuitous route home to Fallujah, steering clear of Baghdad and its omnipresent U.S. checkpoints. The fighters told the officer that their gunrunning trucks often travel with escort cars in front and behind to watch out for Americans and create a distraction if anyone tries to search the truck.

The group's ambushes require a steady supply of ammunition, al-Rawi and his comrades said. A week before the interview, they launched a 60mm mortar attack on a convoy just 10 minutes down the road from the safe house. They fired the rounds from a thickly overgrown field dotted with houses. The attack set a Humvee on fire, according to the men. Four big craters mark the roadside where the ambush took place. That was one of several ambushes the three fighters say their cell has carried out in recent weeks, both in the Fallujah area and around Baghdad's airport. Several attacks have used explosives detonated by remote control, they said, and all have taken place in the early morning or right after nightfall.

The fighters described a simple but apparently effective communications system for coordinating the group's actions. "There is a central command, and we communicate on a daily basis," said al-Rawi. At the top of the organization is a man the fighters call "a high-ranking officer who knows the art of fighting." Couriers deliver his handwritten instructions on paper. The orders often mention specific targets or preferred means of attack. Al-Rawi said the group also has special units that carry out surveillance of U.S. targets. Weapons practice is an almost daily affair, according to the fighters, who all said they were veterans of Saddam's military. All three are members of the same clan in the Dulaimi tribe, which was mistakenly attacked by U.S. forces early in the war. The fighters claim their group has no need of recruiters; they say their neighbors are "begging for weapons" to fight the Americans.

The U.S. military is counting on ordinary Iraqis to help stamp out the insurgency. Whoever wins this battle for hearts and minds will ultimately win the war. The results so far have been mixed. U.S. troops recently withdrew completely from the city of Fallujah after attacks became too frequent and costly. Now the fighters want to repeat that success elsewhere. They have learned to hit Coalition targets with explosives and get away before the Americans can start shooting in all directions. Besides the steady attrition in U.S. lives, every counterattack deepens the people's resentment against the foreigners. And the dislike is mutual. "Too many of our soldiers out there are beginning to hate the Iraqis," says a senior Defense Department civilian.

The fighters seemed able to move openly in Amriyah, without fear that anyone might report them to the Americans. The house was located near a former weapons factory; on Friday afternoon a U.S. military patrol came within a 10 minutes' drive from the place. The neighborhood was drab but relatively affluent by Iraqi standards. "The Americans will go to their funerals here," al-Rawi said. The Iraqis and the Americans alike have already attended too many funerals. But no one has any easy formulas to make the killings stop.