The U.S.-trained Afghan army was responsible for a New Year's Eve attack on a wedding party that killed as many as two dozen people including women and children, local officials have said.
The incident took place in Sagin district in the southern province of Helmand, and was first reported on by the Associated Press.
Witnesses said the house where the wedding was taking place was hit after guests fired celebratory shots into the air.
The incident sparked hundreds of people to travel from Sagin to the home of the governor in the provincial capital to demand justice, AP reports.
"What we know so far is that our soldiers fired mortar rounds from three outposts but we do not know whether it was intentional," General Mahmoud, the deputy Commander of the Afghan 215 corps in Helmand province, told Reuters.
"We have launched our investigation and will punish those who did this," he said.
The exact toll from the attack is still unclear; Agence-France Presse's latest reporting indicates that 17 people, all women and children were killed, and that 49 others were wounded in the shelling. Reuters reported Thursday that 26 people were killed and 41 others wounded.
According to reporting by AP on Friday, two soldiers have already been arrested, and eight more are under investigation. Mahmoud told AP that there was "still a possibility of more arrests."
In a statement issued Thursday, United Nations Secretary-General Ban Ki-moon condemned the attack and urged the Afghan government to conduct a full investigation of the incident.
The deadly fire came at the end of what was the deadliest year for Afghan civilians, and just hours before Afghanistan formally took over security operations for the country, even as the U.S. is continuing its military role there for at least another year.
"I want to congratulate my people today that Afghan forces are now able to take full security responsibility in protecting their country's soil and sovereignty," Afghanistan's President Ashraf Ghani said in a speech Thursday marking the transition.