Brazil's prosecutors have launched an inquiry after photos were published showing soldiers teaching Amazonian Indians how to shoot assault rifles.
Brazil has 700,000 Indians, mostly living in the Amazon rainforest
The newspaper Extra published the photographs it said were from a CD-Rom prepared by the Brazilian military.
Brazil's Indian affairs bureau, Funai, also wants an explanation, saying it is not aware of any training programmes involving indigenous communities.
But Defence Minister Jose Alencar said the training was always voluntary.
Federal Prosecutor Ela Wiecko Volkner de Castilho said she was concerned that Indian civilians were being exposed to firearms.
"It is not right to use civilians, let alone Indians in military exercise," she told Reuters news agency.
"This contact with weapons is harmful for indigenous peoples."
'No better schooling'
She also said the area where it took place is suspected to be near the western border with Colombia. Brazilian Indians are occasionally reported to be recruited by Colombian rebels to serve in their forces.
Funai said it is concerned Indians may be forced into Brazil's military training and recruitment.
Indians are allowed to serve in the army but only if they volunteer. Other Brazilians are required to serve.
Anthropologists say Indians who do join the army find it difficult to return to tribal life afterwards.
The photographs published by Extra show uniformed officers training men and women in everyday clothes how to aim assault rifles in a shooting range.
Defence Minister Jose Alencar said he did not understand "why everyone is so shocked with these spontaneous army training sessions in the Amazon."
"There is no better schooling than that of the Brazilian Armed Forces. It's an extraordinary education," he went on.