BUENOS AIRES (Reuters) - An activist whose disappearance in southern Argentina in August captured the country’s attention in the months before mid-term elections died of drowning and hypothermia, the judge investigating the case said on Friday.

Santiago Maldonado had been missing from Aug. 1. He had attended an indigenous land rights protest in Patagonia that day. His body was found on October 17 in a nearby river.

Federal Judge Guillermo Gustavo Lleral told reporters outside a morgue that an autopsy showed the cause of Maldonado’s death was drowning and hypothermia, and that his body was in the Chubut river for at least 55 days.

Some government opposition and rights groups have said that state security forces took Maldonado, a 28-year-old craftsman, after police reportedly clashed with Mapuche Indians who claim territory throughout southern Argentina and Chile.

The groups’ allegations that President Mauricio Macri’s government covered up Maldonado’s whereabouts overshadowed a mid-term congressional election on Oct. 22. Macri’s government has said there was no evidence that showed security forces had detained Maldonado.

On Friday, Maldonado’s brother said he would keep insisting on an investigation. “This was the cause of death, but we still do not know what happened,” Sergio Maldonado said after a meeting with Lleral.