On Tuesday 10 people were killed and at least 16 wounded in a similar incident.
Residents angered by two days of shootings accused US soldiers involved in yesterday's event of firing deliberately at a crowd of civilians gathered near the governor's building in the city's central administrative district.
But Captain Frank Thorp, a spokesman for US central command in Qatar, vehemently denied the claim. "It absolutely didn't happen," he said. Marines who were securing the governor's building had opened fire, he said, but they were aiming above the crowd at gunmen who were sniping from a rooftop. "The marines were fired upon... away from the crowd," Capt Thorp said. "They fired back, but they never fired at the crowd."
He added that the shooting ended as soon as the marines returned fire: "It had nothing to do with the crowd."
Another version came from Mahmoud Ahmed, a retired Iraqi military general, who witnessed the incident. He said that at the time of the shooting, Mosul police were trying to prevent looters from stealing money from the city's nearby central bank.
He said police had fired a number of times into the air to scatter the looters and that US soldiers opened fire, believing they were under attack.
"I heard three shots from the bank," he said. "Then the Americans opened fire. They sprayed the area. They must have fired 1,000 rounds. But they didn't seem to fire at the bank, they fired in the direction of the crowd by the governor's office."
Amar Ghanem Abdullah, 25, who was wounded in both legs, was among police ordered to stop the looting. He said the police shot in the air to disperse the crowd, and then the Americans fired from the roof of the governor's building.
The Americans "thought we were shooting at them... I don't think they were shooting at us deliberately."
The shooting follows Tuesday's violence in which protesters hurled rocks and fired at US soldiers protecting Mashaan al-Juburi, an Iraqi opposition figure and would-be Mosul governor, who gave an address at the governor's building.
The appearance of Mr Juburi, a former Republican Guard commander, appeared to goad the crowd. Eyewitnesses say the mood of the crowd turned when Mr Juburi hailed the arrival of American forces in Mosul and US troops hoisted an American flag above the governor's building.