A homicide car bomb struck Baghdad's Shiite militia stronghold Saturday, killing at least 18 people as international envoys met in the Iraqi capital to talk about stabilizing the violence-shattered country.

The blast hit an Iraqi patrol in Sadr City at midday, scattering burning debris across a small bridge, witnesses said.

An Associated Press reporter traveling with U.S. troops nearby said the explosion showered shrapnel across a joint U.S.- Iraq security station 300 yards away. The partially shattered windshield of a car landed at the gates of the compound.

Police said at least 18 people were killed and 48 wounded.

Home to about 2.5 million of Baghdad's poorest residents, Sadr City is the base for fighters allied to anti-American cleric Muqtada al-Sadr. His militia, the Mahdi Army, has laid low in recent weeks during a U.S.-Iraqi security crackdown under pressure from Iraqi Prime Minister Nouri al-Maliki.

Hours earlier, Iraqi special forces teams backed by U.S. soldiers detained six suspects believed to be a rogue members of the Mahdi Army, the U.S. military said in a statement.

The suspects were accused of coordinating and carrying out kidnappings and murders of Iraqi civilians, the statement said.

In central Baghdad, two mortars fell near Iraq's Foreign Ministry, where envoys gathered for an international conference on how to quell the violence and bolster Iraq's government. There were no reports of injuries, but smoke was visible from the meeting area.

Meanwhile, Iraqi officials said they were holding a top Al Qaeda official, but not the terror mastermind Abu Omar al-Baghdadi who they believed was captured a day earlier.

"After preliminary investigations, it was proven that the arrested Al Qaeda person is not Abu Omar al-Baghdadi, but, in fact, another important Al Qaeda official," said Brig. Gen. Qassim al-Mousawi, an Iraqi military spokesman.

"Interrogations and investigations are still under way to get more information," he said.

Al-Mousawi declined to give the suspect's name on Saturday.

It was al-Mousawi who announced late Friday that al-Baghdadi had been captured. A senior adviser to the prime minister also had told the AP that al-Baghdadi had been taken into custody. The adviser spoke on condition of anonymity because he was not authorized to release the information.

Al-Mousawi said the suspect at first identified himself as al-Baghdadi, and that his identity was corroborated by another man captured with him. The reported arrest followed rumors this week that al-Baghdadi's brother had been arrested in a raid near Tikrit.

Almost nothing is known of al-Baghdadi, including his real name and what he looks like; his capture would be difficult for officials to verify.

He is believed to lead the shadowy Islamic State of Iraq, an Al Qaeda-inspired group that challenged the authority of Iraq's elected government. He has also signed militant messages posted online, as the leader of the Mujahedeen Shura Council — an umbrella group that includes Al Qaeda in Iraq.

An alleged member of the Islamic State of Iraq was among 27 suspects detained in U.S. raids across Iraq overnight, the U.S. military said.

One suspect was killed and 18 were detained in Taji, an area on the northern outskirts of Baghdad, the military said in a statement.

Eight suspects were captured in Mosul, 225 miles northwest of Baghdad, and one was detained in Ramadi,115 kilometers 70 miles west of Baghdad, it said.

Also Saturday, the U.S. military said it was investigating the shooting of three Iraqis in Baghdad's Azamiyah neighborhood. American paratroopers fired on a vehicle when it failed to respond to warning signals, the military said in a statement. Three Iraqis were killed and three others were wounded in Friday's incident, it said.

In other violence, a roadside bomb killed three Iraqi policemen and wounded another Saturday in central Ramadi, police said.

Gunmen opened fire Saturday on Shiite pilgrims in Latifiyah, about 20 miles south of Baghdad, police said. One person was killed and three were wounded. Later, two more pilgrims were killed in shootings in eastern Baghdad, police said.

The pilgrims were on their way back from a Shiite shrine in Karbala, where millions of faithful were performing rites this weekend for Arbaeen, a holiday that marks the end of a 40-day mourning period after the death anniversary of the Prophet Muhammad's grandson.

Some 340 people, mostly Shiite pilgrims en route to Karbala, were killed in sectarian attacks this past week.