The munitions store in Forward Operating Base Falcon was still burning Wednesday, more than 13 hours after the strike set off a blaze that lit up the night sky and spread panic in the already shell-shocked Iraqi capital.
"Intelligence indicates that civilians aligned with a militia organization were responsible for last night's mortar attack," said US army Lieutenant Colonel Jonathan Withington, spokesman for Multinational Division Baghdad.
Surprisingly, there were no reports of casualties among the more than 2,000 US military and civilian personnel at the camp, nor among the citizens of Baghdad, whose night was disturbed by several hours of terrifying detonations.
"At the beginning we heard an ordinary explosion. We thought the Americans had fired at something, but I went out of the house and saw fire from inside the base," said Abu Sajad, who lives near the base with 24 relatives.
"The explosions got stronger and stronger. Two window panes were broken in my house. Rockets were flying here and there, some of them far away. The fire went down then renewed again. Then there was a big explosion.
"We could not sleep all night. We went into two rooms in the middle of the house for safety. My brother put cotton wool in four of the children's ears. We hugged them to comfort them," he said.
Withington said that engineers and firefighters were working to quell the flames and make the remaining munitions safe.
"The fire ignited tank, artillery, and small-arms ammunition at a forward operating base in the central Rasheed district of Baghdad," he said, explaining that the 82mm mortar round was fired from Abu T-Shir, a Shiite district. "At the time of the attack, base personnel went to full alert. Attack aviation and unmanned aerial vehicles flew overhead in an attempt to locate the terrorists' mortar position," he continued.
"Soldiers and base employees were moved immediately to the safety of hardened buildings and structures on the base," a coalition statement said. "Firefighters from the base and hazardous material experts will continue to work toward extinguishing the fire. Engineers and explosive ordnance detachment teams will begin deliberate clearance of potential unexploded ordnance."
While there were no reports of US casualties, the explosions marked a rare success for mortar teams working for militia and insurgent groups, which do not often cause much damage to well-protected US facilities.
Given the nature of the attack and the direction from which the shell was fired, suspicion will fall on Shiite militia groups such as the Mehdi Army, a loosely organized armed group loyal to radical cleric Moqtada Al Sadr.
The blaze broke out at around 10:40 pm (1940 GMT) Tuesday and shortly afterwards a steady drumbeat of explosions could be heard as far as five kilometers (2.4 miles) away in the city centre.
Government spokesmen appeared on state television to urge calm and quash rumors that the city was under attack.
Iraqi officials said that shells "cooking off" in the fire had crashed down in five areas in the south of the city, but the interior ministry said that there did not seem to have been any casualties outside the base.
State television showed live pictures of a blazing inferno lighting up the night sky from the south side of the metropolis.
US troops in Baghdad are taking part in Operation Together Forward, along with more than 40,000 Iraqi personnel, to quell an eight-month-old outbreak of sectarian bloodletting between rival Sunni and Shiite factions.
Despite the security plan, United Nations and Iraqi officials estimate that more than 100 people are killed each day, more than at any time since the invasion, and Baghdad's streets are littered with corpses daily.
Three car bombs exploded in Baghdad Wednesday, killing four people and wounding 34 more, while police found the corpses of a total of 110 murder victims over the previous two days.
© 2006 Agence France-Presse