In other violence, a suicide bomber detonated a garbage truck packed with explosives outside the Agriculture Ministry and a hotel used by Western contractors on Wednesday, killing at least three people, officials said. The bomber also died.
Two other car bombings were also reported. Police 1st Lt. Mohammed al-Duleimi said one car bomber targeted an American checkpoint outside a base in Habaniyah, 50 miles west of Baghdad. Another car bomb exploded near U.S. troops close to a U.S. base in Abu Ghraib, just west of the capital, police Lt. Akram al-Zubaie said.
No other details were available and the U.S. military could not be immediately reached for comment.
In other developments:
Twenty of the corpses were found late Tuesday in a field near Rumana, a village about 10 miles east of the western city of Qaim, near the Syrian border, police Capt. Muzahim al-Karbouli said.
Each of the bodies had been riddled with bullets — apparently several days earlier. They were found wearing civilian clothes and one of the dead was a woman, al-Karbouli said.
South of Baghdad in Latifiya, Iraqi troops on Tuesday made another gruesome discovery, finding 15 headless bodies in a building inside an abandoned former army base, Defense Ministry Capt. Sabah Yassin said.
The bodies included 10 men, three women and two children. Their identities, like the others found in western Iraq, were not known.
Yassin said some of the dead men in Latifiya were thought to have been part of a group of Iraqi soldiers who were kidnapped by insurgents in the area two weeks ago, Yassin said.
Wednesday's truck bombing in central Baghdad shook nearby buildings in the heart of the capital, injuring dozens of people and covering a huge swath of sky with acrid black smoke. Volleys of automatic weapons fire could be heard before and after the explosion.
Police said a group of insurgents wearing police uniforms first shot dead a guard at the Agriculture Ministry's gate, allowing the truck to enter a compound the ministry shares with the adjacent Sadeer hotel. Guards in the area then fired on the vehicle, trying to disable it before it exploded.
The truck blew up in a parking lot. Several burning vehicles were in flames and around 20 cars were damaged.
The violence came a day after the U.S. military announced it was speeding up an inquiry into the shooting death of an Italian agent killed Friday by U.S. troops at a Baghdad checkpoint — a friendly fire incident that has strained relations with Italy, a key American ally. The agent was escorting Italian journalist Giuliana Sgrena to the airport just after insurgents freed her.
The shooting that killed Italian intelligence officer Nicola Calipari and wounded Sgrena, a 56-year-old journalist for the left-wing Il Manifesto newspaper, angered Italians and rekindled questions about the country's involvement in Iraq.
Italy sent 3,000 troops to Iraq, while Bulgaria has 460 here. Both countries have said they will not withdraw their troops, but domestic pressure to bring them home is growing.
In Rome, Prime Minister Silvio Berlusconi's office said the premier had "expressed the satisfaction of the Italian government" at the accelerated U.S. military investigation. Friendly fire investigations typically take months.
President Bush called Berlusconi on Friday and promised a full investigation into the attack, which took place after nightfall as the car carrying Sgrena, Calipari and two other agents approached Baghdad airport. Another agent also was wounded.
Italian Foreign Minister Gianfranco Finithat U.S. troops killed Calipari by accident, but disputed Washington's version of events.
Fini said the car carrying Calipari and Sgrena was not speeding and U.S. troops did not order it to stop, contrary to what U.S. officials say. But Fini dismissed allegations made by Sgrena that the shooting was an ambush.
"It was an accident," Fini said. "This does not prevent, in fact it makes it a duty for the government to demand that light be shed on the murky issues, that responsibilities be pinpointed, and, where found, that the culprits be punished."
The U.S.-led coalition said a follow-up investigation will be led by U.S. Brig. Gen. Peter Vangjel and will take three to four weeks. Italian officials were invited to participate.
The investigation was ordered by the top U.S. commander in Iraq, Army Gen. George Casey. Vangjel, of the 18th Airborne Corps, commands all Army artillery in Iraq. He arrived in the country in January.
In Washington, Casey said he had no indication Italian officials gave advance notice of the car's route. "I personally do not have any indication of that, even on a preliminary basis," Casey said.
The U.S. 3rd Infantry Division, which controls Baghdad, said the vehicle was "traveling at high speeds" and "refused to stop at a checkpoint."
An American patrol "attempted to warn the driver to stop by hand and arm signals, flashing white lights, and firing warning shots in front of the car," it said. "When the driver didn't stop, the soldiers shot into the engine block which stopped the vehicle, killing one and wounding two others."