A road crash that triggered deadly anti-foreigner rioting in Kabul occurred because a military truck lost its brakes coming down a hill and plowed into a line of cars, the U.S. military said Tuesday.

A gunman riding a motorcycle on Tuesday, meanwhile, shot and killed three Afghan women working for an international aid group and their driver in northern Afghanistan, said Gov. Jama Khan Ahmdar of northern Jawzjan province.

Rioters on Monday stoned the U.S. convoy involved in the accident, then headed to the center of Kabul, ransacking offices of international aid groups and searching for foreigners while chanting "Death to America!" Smoke billowed from burning buildings.

The death toll from the unrest rose to 11, with most dying from gunshot wounds, according to three city hospitals. More than 100 people were wounded.

Up to five people were killed in the accident, but it wasn't clear whether these deaths were among the tolls the hospitals reported.

CountryWatch: Afghanistan

Military spokesman Col. Tom Collins said the truck's brakes "apparently overheated and failed" as it came down the long hill.

"The driver, very experienced in the operation of this type of vehicle, a heavy cargo truck, applied the primary and emergency brakes and took evasive action to avoid hitting pedestrians," Collins said.

The truck hit several unoccupied parked cars in an effort to slow down, but the truck hit occupied vehicles at an intersection anyway, he said.

"We extend our deepest condolences to the families of those killed and injured in this unfortunate traffic accident," Collins said.

The crash sparked the worst riots across Kabul since the fall of the Taliban in 2001. Hundreds of Afghan and coalition troops took up positions around the capital Tuesday to prevent further unrest, and the city of 4 million was calm as stores reopened and residents went to work.

Many expressed dismay as they surveyed the damage from Monday's riots.

"Where were all the security forces yesterday?" asked Asadullah Chelsea, who owns a supermarket popular with foreigners. "I have lost thousands of dollars of stock."

The rioters in Kabul on Monday claimed U.S. troops had shot and killed civilians at the scene of the accident.

A spokesman for the U.S.-led coalition confirmed there was gunfire but said coalition personnel in one military vehicle only fired over the crowd. The coalition expressed regret for any deaths and injuries, and said there would be an investigation.

The three female employees of ActionAid International and their male driver died when a gunman pulled alongside their vehicle and fired into it with an assault rife, said Ahmdar, the provincial governor.

Officials of the Johannesburg, South Africa-based agency had no firm information on why the employees were targeted, but said it could have been because the women were breaking with tradition by working outside the home.

Kate Nustedt, an ActionAid spokeswomen, said by telephone from South Africa that the slain women had worked for the group for about one year, helping educate Afghan women about their rights and how they country's new democratic processes work.

President Hamid Karzai went on television Monday night to decry the violence in Kabul, branding the rioters as troublemakers who should be resisted.

About 2,000 troops prowled the city to enforce an overnight curfew, which passed without incident, said Gen. Zahir Azimi, the Defense Ministry spokesman.

"The army has control of the city. We have tanks in the city for the first time," he said.

As the violence eased late Monday, embassies sent out convoys of armored vehicles to pick up their nationals from homes and buildings and bring them to foreign military bases where they spent the night.

Afghans often complain about what they call the aggressive driving tactics of the U.S. military. Convoys often pass through crowded areas at high speed and sometimes disregard road rules. The U.S. military says such tactics are necessary to protect the troops from attack.

Patience with the 23,000 U.S. soldiers and other foreign troops in Afghanistan is also fraying over recent deaths of civilians, including at least 16 people killed by an airstrike targeting Taliban fighters in a southern village last week.