Alvaro Lara, the Narino state prosecutor, said the unidentified attackers were looking for a woman called "The Matron", apparently so that she could answer a debt.
"Seconds later, the armed men began to shoot at anything that moved," Lara said.
In February, fighters for the Revolutionary Armed Forces of Colombia (Farc) said that they had killed eight Awa at a reservation for allegedly informing the Colombian army as to the group's movements.
Narino state contains dozens of coca plantations, with several illegal armed groups - from left-wing organisations to far-right militias – processing the leaf into cocaine and trafficking it out of the country.
Those who survived the gun attack said that the killers were tall, fair-haired men with mustaches, ruling out local Indians, Navarro said.
About 20,000 Awa live in Colombia, a country that is home to more than one million members of at least 80 indigenous communities.
At least 75 Indians have been killed in Colombia this year as a result of the continuing war between various armed groups.
Luis Evelis Andrade, the president of the National Organisation of Indigenous Colombians, said recently that indigenous groups are often caught up in battles that are not of their making.
"The lands they [the authorities] gave us - which are the most inhospitable - are today in dispute by armed groups," he told the Associated Press, adding that the reserves are often chosen by armed groups to cultivate coca.