Of the four car bombs that exploded in Iraq on Monday, it was the concussive detonation on Baghdad's crowded Saadoun Street that left Muhammed Ahmed suddenly all by himself in his third-grade classroom.

"All the students ran away," said Muhammed, 8, who, though frightened, made his way with his book bag to the scene of the explosion "to see what happened."

A pickup truck loaded with explosives had barreled into a convoy of sport-utility vehicles, which have become the most conspicuous targets in Baghdad because they invariably carry foreigners. The truck's charred chassis littered the street, along with gnarled hunks of metal and rubber, the shining gravel of burst window glass, a human head and other body parts.

"I always fear when I see the explosion scenes on TV, but now I see it live," Muhammed said. "I didn't know it would be this scary."

Across Iraq on Monday, car bombs killed at least 22 people and injured more than 100. The car bomb on Saadoun Street, outside the Baghdad Hotel, killed six and injured 15, and was the second bombing there in the past year. So was the bomb that killed 15 and wounded 81 at a National Guard recruiting station on the stretch of road just outside Baghdad's Green Zone where the head of Iraq's former Governing Council was killed in May, and close to the U.S. Embassy and key government installations.

Monday's third blast, in the northern city of Mosul, killed a bystander and the two bombers, who apparently blew themselves up by accident while waiting behind an elementary school for a U.S. military patrol to pass by. Yet another suicide bomber struck a U.S. patrol elsewhere in Mosul, injuring one American soldier.

"Hour by hour, the situation is getting worse," said Saad Muwaffaq, 25, who was guarding a building about 50 yards from the Saadoun Street blast. "We cannot walk for 100 meters without fearing a car bomb now."

Two American soldiers were killed Sunday by small-arms fire at a Baghdad checkpoint, the U.S. military. A senior official at Iraq's Ministry of Science and Technology was assassinated in Baghdad, Col. Adnan Abdul-Rahman, an Interior Ministry spokesman, told the Associated Press. Another Science Ministry employee was also killed.

One U.S. soldier was killed and two others were injured late Monday when a roadside bomb exploded beside their convoy near Baghdad, the military said.

The al-Jazeera satellite television network reported the release of a video showing two hostages -- one an Iraqi businessman based in Italy, the other a Turkish national -- being shot in the head by guerrillas.

Car bombings in Iraq have become commonplace in recent weeks; U.S. officials counted more than 70 during September.

The people killed, more than ever, are Iraqis. The aftermath of the Saadoun Street bombing brought home the horrors that may have become routine to a world watching from a safe distance but that in Iraq still pierce individual lives as deeply as ever.

"Why are they doing these attacks? Is it forbidden for Iraqis to live normally?" Khairiya Abdul Hussein, 51, cried as she searched for her daughter, who worked as a maid for a company on Saadoun Street. "I always asked her to quit. We know this street is a target, but she didn't listen."

Jafar Zyara, 17, who sells tea on the street, said he saw the convoy of SUVs emerge from an alley beside the Baghdad Hotel, which is known as a residence for foreign private security firms and intelligence personnel. Iraqi guards halted traffic to make way for the convoy, which carried members of DynCorp, a Reston-based defense contractor. News services said the bomb was hidden under a load of dates in the pickup.

"A woman came to me 10 minutes before the explosion and had tea," Zyara said. "After, I went near the scene to look for her, but I found nothing but her scarf on the ground full of blood."

The losses are counted not only in human lives. The blast undid the renovation of the Manhal restaurant in the al-Kuwait Hotel, a small enterprise unprotected by the type of towering blast walls that gird the Baghdad Hotel.

"If the situation continues like this, I will not stay longer in Iraq," said Abdul Adhim Hussein, 44, an Egyptian who was working on the renovation. "If I build or renovate something, the next day it will be demolished by a car bomb.

"People try to rebuild their country, but the terrorists give them no chance."

William B. Taylor Jr., head of the new Iraqi Reconstruction Management Office, confirmed on Monday that insecurity has greatly retarded efforts to spend the $18.4 billion that Congress appropriated to rebuild the country.

For example, of 21 water treatment plants under construction in Iraq, Taylor said, one has been completed. Its official opening in a south Baghdad neighborhood last Thursday was devastated by two car bombings. Among the dead were 35 children, a wrenching toll that one Iraqi official said "pushes very hard at the limits of barbarity."

"I fear to go to school now," said Muhammed, the third-grader taking in the sight of body parts on Saadoun Street. "What if this happened to me on the way to school? What if my mother couldn't recognize me when I am laying on the ground? I'll be left alone on the street."

A couple of miles away, Dhia Abbas, 24, stood amid a flurry of activity in the emergency room at Yarmouk Hospital. His face was bloodied by shrapnel from the car bomb detonated near the National Guard post where he had reported for his first day of work. It was also his last, he said.

"This is it," said Abbas, who had worked in security for the government of deposed president Saddam Hussein. "I am never coming back to that place. I may starve, but I am not going back."

Not everyone, however, was dissuaded by the day's violence. Razaq Hadi, a postgraduate student at Baghdad University, was in a minibus on his way to school when the Saadoun Street blast killed the bus driver and two passengers. He felt the bus leap into the air, and escaped only by climbing out a window.

"I escaped death today," said Hadi, 36. "That means I have another chance in life.

"So does Iraq," he added with a smile. "It has another chance in life, and we will use it."

Special correspondents Khalid Saffar and Bassam Sebti contributed to this report.

A foreign national is evacuated after a suicide bombing in Baghdad that targeted SUVs carrying employees of DynCorp, a Reston-based defense contractor. Six people were killed and 15 were injured in the attack.