The company managing the project said it was withdrawing employees from southern Afghanistan, according to the U.S. State Department.
The victims were transporting the body of one of five victims in the earlier assault to the capital, Kabul, for a funeral when militants stopped their vehicle and shot everyone in the head, said Naik Mohammed, a doctor in the town of Qalat where the bodies were taken.
The killings are the latest spasm of violence in a region prey to drug traffickers and insurgents. The assaults, along with the kidnapping of an Italian aid worker this week in Kabul, have raised fears that tactics used in Iraq may be copied here.
Also in the country's south, two U.S. soldiers were slightly wounded when a bomb exploded near their vehicle, the U.S. military said.
Two of the Afghans killed in Thursday's attack and all five killed Wednesday were believed to be employees of Chemonics International Inc., a Washington-based consulting firm. Two others killed Thursday were relatives of one of Wednesday's victims, and the remaining two were drivers.