Three lives, an ordinary summer day in Baghdad–and then, in an instant, everything changed. At 4:37 p.m., the deadly load inside the truck blew up: 1,000 pounds of Iraqi ordnance, including mortar rounds, artillery shells and hand grenades, packed around a 500-pound bomb. The explosion killed 24 people, including Vieira de Mello and everyone in the room with him, seriously injured 86 more–and left the shellshocked survivors grieving and struggling to comprehend the enormity of the catastrophe.

The men and women who worked inside the Canal Hotel were a special breed. Palestinian, Japanese, Canadian, Somali, Trinidadian–many of them had worked and lived together in close proximity in some of the world’s most dangerous places. They were people like Marshall, an unmarried nomad who had spent much of his adult life hopscotching from combat zone to combat zone, hooked on the adrenaline rush of Third World chaos and driven by a desire to change societies. Others, like Adomian, were locals who’d found refuge, during Saddam’s time, in the cozy familial atmosphere of the United Nations.

All of them were aware of the risks: “The first day I arrived, the U.N. security team told us, ‘We are a soft target, and if they want to take us out, they can’,” Marshall told NEWSWEEK. Yet all assumed that their status as civilian do-gooders offered them protection. Now those illusions lie buried in the rubble that crushed Sergio Vieira de Mello and some of the United Nations’ brightest and best.

Aida Moses Adomian, 42, viewed the United Nations as her second family. She was devastated when her foreign colleagues pulled out of Iraq on March 18. “I wasn’t scared of the war, but of what would happen afterward,” she says. After huddling in her house for three weeks with her sister, two brothers and aging mother, she emerged to find looters picking U.N. headquarters clean. “Cars, desks, computers, air conditioners–they took everything,” she says. In mid-April she returned to work in the empty shell of the building, taking an inventory of what was lost. Her boss returned in May; slowly the building came back to life. For Adomian the Canal Hotel became an oasis–from the thieves who stole her car at gunpoint, from the blackouts that left her family sweltering in the summer heat.

Sergio Vieira de Mello didn’t want to come to Baghdad. The 55-year-old Brazilian career diplomat had spent the past year based in Geneva as the United Nations’ high commissioner for human rights, and he viewed nation- building as a distraction. But Vieira de Mello had served with distinction as special envoy to Kosovo and East Timor, and U.N. Secretary-General Kofi Annan begged him to take on the Iraq job. Born into a wealthy Rio de Janeiro family, educated at the Sorbonne, he spoke English, French, Italian, Spanish and Portuguese fluently. He cut a dashing figure as he shuttled between hellholes, conferring with warlords and politicians, always immaculately attired. “Sergio had an incredible energy; he was physically fit, jogging many miles [each] week,” remembered Jose Ramos-Horta, East Timor’s foreign minister. “He was a patient diplomat, a good listener, an effective communicator. Above all else, he was a consensus-builder.”

Vieira de Mello flew into Baghdad on June 2. His assignment was to last only four months, and he was eager to get an Iraqi government up and running. Crisscrossing the country in a U.N. plane, from Basra to the holy Shiite city of Najaf to Erbil in Kurdistan, he and his team identified civic and religious leaders, enlisted their cooperation and offered support. Vieira de Mello’s most delicate task was establishing a relationship with L. Paul Bremer, the chief U.S. administrator in Iraq. Though cordial in public, the pair squabbled over many aspects of nation-building: Bremer wanted the Iraqi Governing Council to be an advisory board, while Vieira de Mello insisted it have real power. “We worked nonstop from 7:30 in the morning until 7:30 at night, and then continued when we returned to our hotels,” says Marwan Ali, Vieira de Mello’s political adviser. “The phones didn’t work, meetings were delayed, but it was incredibly exciting. We felt we were achieving something.”

David Marshall was one of many dedicated young reformers who were drawn into Vieira de Mello’s orbit. Born in Oakville, Ontario, Marshall earned a law degree at Leeds University and a master’s at Harvard. He served a stint in war-racked Sierra Leone as a trial observer and later helped rebuild the courts in Afghanistan and Kosovo.

In May 2003, Marshall joined Vieira de Mello’s human-rights team in Geneva; on Aug. 6 he flew to Baghdad to conduct a two-week assessment of the country’s judicial system. Marshall, 40, had spent plenty of time in unstable countries, but Baghdad felt especially dangerous. He supported the decision not to surround the headquarters with tanks and barbed wire–“We didn’t want to be cut off from the people we were trying to help,” he says–but the risk was obvious. “We were vulnerable,” he says, “and we knew it.”

Even so, Marshall spent long days riding through Baghdad in a dark-blue U.N. Toyota Land Cruiser, visiting dilapidated courthouses, interviewing judges, prosecutors and attorneys, looking for the rule of law but rarely finding it. Apathy and corruption were rampant; those hauled before the courts were almost always found guilty, unless they could manage a hefty bribe. “I asked one judge how long his longest trial had lasted, and he replied, ‘Two sessions.’ ‘How long was each session?’ I asked. He said, ‘Half an hour’.”

Tuesday, Aug. 19, was another stifling day in Baghdad. At 8:30 in the morning, under a searing sun, Adomian climbed into a colleague’s Land Cruiser and set out for headquarters. They followed a road that runs alongside a fetid canal lined with date palms, then turned left through the main gate. A handful of American troops milled about near the entrance. As always, an Iraqi security team checked their badges and inspected their vehicle, then waved them through the gate.

Adomian strode past the glass-walled cafeteria and into the busy lobby. She turned left and walked through a corridor to the office she shared with two fellow clerks, a Sudanese-Canadian and an Iraqi. The three were involved in a grim task that morning: the United Nations had begun phasing out its Oil-for-Food Program, and they had to sell off all the office’s computers and vehicles; Adomian would be out of a job in November, and wasn’t sure she’d find another. In the early afternoon her younger sister, Shaki, came by the office. A female Iraqi colleague named Lee stopped in just before 4:30. Adomian was impressed by how much weight Lee had lost, and as the trio gathered around the coffee machine, Adomian begged her for details about her new diet.

David Marshall left his room at the Petra Hotel in central Baghdad at 9 o’clock and headed out for another day in Baghdad’s judicial quagmire. Sitting in the back of a dingy courtroom, he watched in frustration as the judge adjourned five straight trials because the Coalition forces had failed to bring the defendants to the courthouse. After the court closed for the day, Marshall headed to the Republican Palace for a meeting with Iraqis and Americans about the progress of war-crimes investigations. Suffering from diarrhea, his pockets stuffed with toilet paper, the attorney returned to U.N. headquarters at 4 o’clock and walked briskly down the hallway to the office of a colleague to hash out a first draft of his report.

Sergio Vieira de Mello was weary. He had spent much of August shuttling around Iraq and visiting neighboring countries. Now he was back in Baghdad, embroiled in another dispute with Bremer. A lot of civilians were getting killed, including a TV cameraman for Reuters. Vieira de Mello summoned his chief U.N. spokesman, Salim Lone, and told him to draft a public statement condemning U.S. military recklessness. He planned to release it that evening.

At 2 o’clock, Marwan Ali ran into his boss in the third-floor corridor. “He said, ‘Marwan, are you still going on holiday this weekend?’ I nodded; he flashed me a smile and told me, ‘Try to relax and come back refreshed’.” Two hours later Vieira de Mello called his closest aides to his office to brief Arthur Helton, 54, a St. Louis-born lawyer and human-rights activist who ran a program on conflict resolution at the Council on Foreign Relations, a New York-based think tank. They sat on brown-leather couches around a low glass table arranged with tea, coffee and bottles of mineral water.

Somewhere in Baghdad, a lone assailant hunched behind the wheel of a Russian-made military truck and drove purposefully through the late-afternoon traffic. According to investigators, he was probably a Saddam Hussein loyalist yearning for revenge against anyone supportive of the American occupiers. (A previously unknown group calling itself the Armed Vanguards of the Second Mohammed Army claimed responsibility on Thursday.) The bed of his vehicle was packed with munitions gathered from some of the dozens of Saddam-era arms bunkers scattered across the country. The FBI suspects that the driver had been given detailed information about the layout of U.N. headquarters from a member of the Iraqi security staff–many of whom were holdovers from the Saddam regime. The driver turned onto an unguarded paved road that runs past the southeast side of the U.N. headquarters. He pulled to a stop below Vieira de Mello’s office. Some witnesses say they watched the attacker try to climb onto the road; others say he made no attempt to leave the cab. Whatever the case, just after 4:30, as Vieira de Mello and his team chatted, unsuspecting, the bomb went off.

Marshall, on the opposite side of the building, was sitting with his back to a wall, speaking with his co-worker. “Do you feel you’ve gotten enough information [about the courts]?” she asked him. At that moment the explosion threw them to the floor. Marshall lay there, ears ringing, choking on dust and smoke. For a few seconds, all was silent. “My God,” he thought, gazing up at a tangle of steel retaining rods and crumbling concrete that had once been the ceiling. “I’m alive.” He groped his way in the darkness down the corridor. People staggered past him, wailing and covered in blood. Marshall reached into his pockets and offered them toilet paper to wrap their wounds. He stumbled down the stairs and straight into the bright lights of a half-dozen TV cameras wielded by journalists who had been attending a press conference on the ground floor.

Turn those f—ing cameras off!" he shouted. Marshall drifted through the ruined lobby, tripping over chunks of concrete, staring in disbelief at bodies that lay partly covered by the wreckage; some of them appeared unscathed, perhaps killed by shock waves from the blast. “Oh, God,” he heard himself saying aloud. “They’re all dead.” A man wandered past him with half his face ripped away; a teenage U.S. soldier, wearing only black gym shorts, a T shirt and a helmet, stood shellshocked amid the carnage.

Survivors began to stream out of the building. Medics and security men dragged out the dead. “Where’s Sergio?” somebody screamed. Outside, Marshall stared in horror at a smoking bomb crater and the pancaked remains of Vieira de Mello’s office. He clambered atop the eight-foot-high pile of rubble. “Help me get him out!” he yelled.

Adomian doesn’t remember the moment when the bomb went off. “Suddenly I felt dizzy,” she says. “Then I could not open my eyes.” When she regained consciousness, she noticed that her windows had been blown out; her wooden desk had broken in half and had been upended by the blast. She was flat on her back, racked with pain, coughing up blood. A few feet away her sister, Shaki, lay unmoving, facedown on the floor. Her colleague Lee was tangled beneath Shaki. She heard the voice of her Sudanese co-worker calling, “Aida, are you still alive?” “Ali,” she replied, “please come take care of my sister.” Adomian called Lee’s name repeatedly, but the woman didn’t answer.

Then she lay there, her head throbbing, bleeding profusely from deep cuts in her arms, waiting for rescue. Two colleagues gingerly carried her outside and laid her down in the grass, alongside both the injured and the dead.

Sgt. Andy Wilson of the Second Armored Cavalry Brigade was riding past the U.N. compound in a Humvee, a routine patrol, when he saw a brilliant flash of light and was knocked backward by the explosion. A black mushroom cloud rose over the blue U.N. flag that fluttered above the headquarters’ arched entranceway. “I knew instantly it was an act of terror,” he says. Wilson, 25, from New Orleans, jumped out of the vehicle and dashed through the gate, thus becoming one of the first U.S. soldiers to arrive at the scene. “It was chaos,” he recalls. U.N. medics had already begun piling corpses outside the building; Wilson helped secure a perimeter, kept the press out and chased back looters. He was well aware of the dangers in Baghdad: in June, his convoy was hit by a roadside mine that killed a comrade, and he had driven the body to the morgue. But this bomb blast was different. It was, he says, “the worst thing I’ve ever seen.”

The mangled bodies of Sergio Vieira de Mello and his advisers were pulled from the wreckage late that afternoon. Nineteen more victims, both foreigners and Iraqis, would follow. David Marshall caught a ride back to the Petra Hotel, shirt streaked with bloodstains, and hastened his departure to Geneva. “I won’t come back,” Marshall said as he carried his suitcases into the lobby on Wednesday. “I don’t think I’ve got the balls. I don’t know who the hell wants to come to Baghdad now.” Across town at Baghdad Teaching Hospital, Adomian lay in pain in a stifling 11th-floor ward. Her head was swathed in bandages; red welts and puncture wounds covered her right arm, and she was still spitting up blood from her injured lungs. Forty hours after the attack, her sister remained missing; she had last been seen on a stretcher in a U.S. military ambulance. Adomian’s Iraqi colleague Lee had died at the scene. Like Marshall, Adomian couldn’t imagine returning to work for the United Nations. She felt trapped in Iraq. “I’d go to Armenia if I could,” she said weakly, as her brother dabbed a bruised eye with a wet washcloth. “The whole country is destroyed, looted. Look at what the Americans have brought us.”

Two days after the bombing, Sergeant Wilson led a NEWSWEEK correspondent and photographer through the wreckage of U.N. headquarters. Wilson trod gingerly through the gutted cafeteria, littered with shards of glass, overturned chairs and the remains of a chocolate birthday cake. He stepped through a jagged hole in the lobby’s rear wall and onto the slabs of concrete under which so many had died. The U.S. military sniffer dogs had finished their work for the day; the body of an Iraqi U.N. employee was believed to be still buried in the rubble. A team of sappers perched at the edge of the sheared-off section of offices, stringing explosive charges to bring down a slab of roof that hung suspended by a spider’s web of steel reinforcement cables. Iraqis watched them from a nearby pedestrian bridge over the canal. The soldiers and journalists retreated to a safe distance. Next came a terrifying flash, a boom and a puff of smoke–a stage-managed blast that gave only the barest hint of the real bombing’s horror.