A bomb tore through a minibus during morning rush hour Monday in a mainly Shiite area in Baghdad, killing at least nine people and wounding 24, Iraqi officials said.

The blast was a grim reminder of the major challenge facing Iraqi forces three weeks ahead of the June 30 deadline for U.S. troops to withdraw from urban areas.

The bomb was attached to the minibus in the southern area of Abu Dshir, a Shiite enclave in the mainly Sunni neighborhood of Dora, police said.

"A ball of fire rose into the sky. We saw a minibus thrown about five meters (yards) into the air, then come down in flames," said Omar Abdul-Ghafar, a university student who was waiting with his friend for another bus.

The explosion left a crater at the entrance of the bus station where commuters were gathered to catch rides to different parts of the city.

An Associated Press photographer saw the charred hulk of the minibus and three other burned-out cars. Security forces sealed off the area while ambulances rushed the wounded to the hospital.

Police and hospital officials gave the death toll and said 24 people also were wounded. An Interior Ministry official said all those killed had been passengers on the bus while the wounded were bystanders waiting nearby.

The officials spoke on condition of anonymity because they weren't authorized to release the information.

Both districts have faced brutal sectarian bloodshed in past years but have seen a sharp decline in violence following a Sunni revolt against insurgent groups such as Al Qaeda in Iraq and a Shiite militia cease-fire.

U.S.-Iraqi forces also increased their presence and cordoned areas off with concrete walls and checkpoints in the citywide push to quell the violence that pushed the country to the brink of civil war.

With the decline in violence, Iraqi authorities have taken down many of the concrete walls in a bid to restore a sense of normalcy in the capital.

But several recent high-profile bombings have raised concerns about the readiness of Iraqi forces to take over their own security.

Abdul-Ghafar, a Sunni resident who fled the violence but returned to the area about six months ago when the situation seemed to improve, said the area was especially crowded because of students planning to take final exams.

His friend was injured in the shoulder and soaked in blood. Lecture notes, cigarette packs and candy bars were scattered on the ground.

"Some children were crying and running aimlessly, looking for their parents," he said. "People were so upset with police and began shouting insults on police and government for the security violations and for removing the concrete walls and stopping the searching process."

The June 30 withdrawal date was provided for in the U.S.-Iraq security agreement that took effect this year. President Obama plans to end U.S. combat operations by September 2010 and remove all U.S. troops from the country by Dec. 31, 2011.

Iraq's Shiite-led government insisted on a timetable for the withdrawal during last year's negotiations that produced the security agreement.

The Iraqi government spokesman, meanwhile, said four Americans, not five, have been detained in an investigation into the killing last month of a U.S. contractor in the Green Zone.

The Americans are believed to be the first taken into Iraqi custody since the security agreement went into effect this year. It removed immunity from Iraqi law enjoyed by private U.S. contractors since the collapse of Saddam Hussein's regime in 2003.

Ali al-Dabbagh said four Americans and one Iraqi were picked up Wednesday during a raid on the Corporate Training Unlimited house. That raid was ordered after Iraqi security forces received information that a suspect involved in last month's stabbing death of contractor Jim Kitterman might be there, he said.

The raiding party found the contractors' weapons permits and operating license had expired and took them into custody, according to al-Dabbagh.

It was unclear if the purported suspect in Kitterman's death was in the house when the raid occurred.