BAGHDAD, Iraq, July 17 - Even in Iraq, where shocking killings have become part of daily life, some acts are so profoundly violent that the country seems to pause, trying to fathom what happened.
That was the case on Sunday, after a suicide bomber appeared in Musayyib, a poor town just south of Baghdad, and blew himself up under a fuel tanker on Saturday night, igniting a fireball that engulfed cars, shops and homes. At least 71 people died; 156 were wounded. Some bodies were badly charred, making identification difficult.
Some senior elected officials and civic and religious leaders spoke out on Sunday, condemning the attack, one of a wave of suicide bombings that has shaken the greater Baghdad area in the past eight days. Grand Ayatollah Ali al-Sistani, the most revered Shiite cleric in Iraq, asked the government "to defend this country against the mass annihilation," according to Vice President Adel Abdul Mahdi, who led a delegation that visited the ayatollah on Sunday.
But nothing has stopped the bombings. In Baghdad, four suicide bombers struck within a span of four and a half hours on Sunday.
An Interior Ministry official listed the attacks: at 8 a.m., a car exploded at a checkpoint guarded by ministry commandos, killing three of them and wounding 10 civilians. Another car bomb killed a police commando and a civilian and wounded five civilians at 9:45 a.m. A third detonated near an office of the High Electoral Commission in Baghdad at 11:10 a.m., killing three and wounding one. The fourth struck at 12:30 p.m., killing one civilian and wounding another.
The surge of suicide attacks torturing the capital has seemingly confounded Iraqi and American forces, which have focused their Baghdad security efforts on stopping the bombers. Attacks are often undetectable until the last seconds before detonation, especially in the case of moving car bombs, and nearby civilians can slow the reaction of security forces.
Additionally, no obvious pattern has appeared in the recent string of attacks except that, like the scores of others that have made suicide bombs a prominent feature of this war, they have often singled out Shiites in large numbers or Iraqi and international security forces.
The National Assembly called for three minutes of silence nationwide on Wednesday to commemorate the Musayyib victims, as well as those in a suicide attack last Wednesday in Baghdad that killed more than two dozen people, mostly children.
Early leads in the investigation into the Musayyib attack suggest that insurgents had carefully planned it for maximum civilian casualties.
Several days earlier, the truck, which belonged to Iraq's Oil Ministry, had been hijacked by armed insurgents and the driver kidnapped en route from Baghdad to Falluja, according to an official at the Interior Ministry, who requested anonymity for fear of administrative punishment or reprisals from the insurgents.
"The only explanation the Interior Ministry has now is that the whole operation was arranged, and an insurgent was waiting in Musayyib to blow himself up at the location," the official said in a telephone interview.
"The situation is not good, and not what we hoped for," Hussein al-Shaalan, a member of the National Assembly, said in a telephone interview. The National Assembly intends to question the interior and defense ministers in the coming days about ways to improve the nation's domestic security.
Since the insurgency coalesced in the months after the ouster of Saddam Hussein, the level of guerrilla violence in Iraq has been cyclical. Mindful that rebel activity sometimes spikes in the weeks or days leading up to specific political events, including elections and transfers of power, military officials predict a rise in insurgent attacks between now and the mid-August date set for the constitutional convention, and again before the mid-October constitutional referendum and in advance of the national elections in December.
The current guerrilla campaign comes several weeks after the American authorities announced what they said was a successful effort to severely reduce the ability of insurgents to launch attacks in the capital - timing that suggests that the bombings may be a response to the American command's claims.
The attacks also come as most of the country's major Sunni Arab communities have begun to coalesce around a commitment to get out the Sunni Arab vote in the December elections for a full government, a decision the government views as a further step toward solidifying a political process that the insurgency has been trying to undermine.
Military analysts say that although it is difficult to assess the insurgency's organization, they believe that the attackers have a flat structure without complicated hierarchies. Much of the violence is carried out by independent cells that may communicate on certain occasions with high-level leaders but mostly take broad guidance from Web sites and videos, and set up their own attack schedules within their own regions.
According to military analysts, insurgents have learned in recent months that while attacking Iraqi or the American-led coalition forces has a certain propaganda payoff, it is much easier to strike civilian targets that have far less protection than troops or installations. And such attacks have the side effect of sowing chaos and distrust within the government.
Nabeel Muhammad, senior lecturer in international relations at Baghdad University, said in a telephone interview on Sunday that the insurgency was "desperate to start a sectarian unrest in the country."
"They keep looking for new methods to attack, and the Iraqi people are the only victim."
The insurgents have shown a keen ability to adapt and strengthen their tactics. Saturday's attack in Musayyib may be a case in point.
Adnan Ahmed, 35, a local government official who saw the bombing, explained that a man wearing a body belt of explosives dove underneath a propane fuel tanker and detonated himself shortly after 8 p.m. as the street teemed with pedestrians, including Shiite worshipers heading to the mosque and shoppers in the nearby market.
The explosion appeared to blow a hole in the bottom of the tank and ignite the fuel inside, officials say. Yet the resulting fireball left the huge fuel cylinder mostly intact.
All night, rescuers and relatives rushed survivors to hospitals around the region and carted bodies to morgues, and others battled the fires that gutted buildings and left behind a junkyard of blackened cars.
By late Sunday afternoon, an eerie, haunting quiet descended over the site. Few spoke at all.
It was the most deadly suicide bombing since Feb. 28, when a driver detonated his sedan full of explosives in a crowd of Iraqi police and army recruits in Hilla, killing at least 122 people.
Despite the huge toll in recent attacks, the Iraqi police have scored two victories against suicide bombers in the past week, capturing two men in the midst of separate suicide attacks.
On Thursday, Iraqi guards at a checkpoint outside the Green Zone in central Baghdad captured a suicide bomber who was part of a triple suicide attack.
And on Saturday, during a funeral for children who died in the bombing on Wednesday, a unit of the Iraqi police stopped a suspicious-looking man approaching the funeral procession and discovered that he was wearing a suicide vest filled with explosives and ball bearings, the American command reported Sunday.
An explosives team disarmed the man, a Libyan, and no one was hurt, according to the American military.
"The bomber was high on drugs and is being treated for the potential overdose," said Col. Joseph DiSalvo, an American commander. The bomber, he said, "came here to kill the grieving parents of the children who were killed on Wednesday."
"I cannot imagine a worse crime."