Officials said on Saturday the miners were possibly killed in clashes with Wide Belt natives, who live on the Roosevelt reserve in Rondonia state.
Brazil's National Indian Foundation (Funai) said tensions have been rising for some time between illegal miners and so-called Indians (natives) on the reserve, considered one of the biggest diamond regions in South America. Mining on Indian reserves is illegal in Brazil.
"I have information that there are a high number of dead miners. I heard in the region of 20," Funai president Mercio Pereira Gomes said.
Mauro Sposito, head of the Brazilian federal police's Amazon drugs unit, also estimated the dead at about 20.
But Brazilian state news agency Agencia Brasil cited Rondonia police as saying 26 dead miners have been found.
The Roosevelt native reserve has been a point of conflict for years between indigenous people and illegal miners who periodically are pushed off the reserve by authorities only to move back in again in search of a fortune in diamonds.
The worst previous confrontation between miners and Indians in recent years took place in 1993 when 16 natives, including women and children, were killed by the miners.