BUENOS AIRES (Reuters) - The family of a young protester who went missing in Argentina nearly three months ago identified his body on Friday, said a brother of the dead man to journalists after leaving the morgue.  Investigators discovered a body on Tuesday in the Chubut River in Patagonia, near where Santiago Maldonado was last seen at an indigenous rights protest on Aug. 1.   We recognized Santiago s tattoos so we are convinced it is Santiago,  said Sergio Maldonado, the brother, after the family identified the body in Buenos Aires.  Some government opposition and rights groups have said 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 allege President Mauricio Macri s government covered up Maldonado s whereabouts. Macri s government says there is no evidence showing security forces detained him.  The disappearance and death of Maldonado has overshadowed Sunday s midterm congressional election in a country where potential cases of abuse by security forces are particularly sensitive.  Argentina s 1976-83 military dictatorship secretly detained, tortured and killed people in clandestine prisons, and rights groups say up to 30,000 people  disappeared.   The judge overseeing the Maldonado case said an autopsy completed late on Friday showed no signs of injury on the body.   Now the cause of death needs to be determined,  Judge Gustavo Lleral told journalists.  Former leftist President Cristina Fernandez, who is seeking a Senate seat in Buenos Aires province, has criticized the government s handling of the case, and mass protests and social media campaigns have demanded to know what happened to Maldonado.  The president s office said on Friday that Macri had contacted Maldonado s mother to offer his condolences.  Mourners left candles, flowers and cards outside the morgue in Buenos Aires, while protesters gathered at the Plaza de Mayo.  