BUENOS AIRES — For two months, a shaggy, solemn face has been plastered on the front pages of Argentina’s newspapers, printed on T-shirts and spray-painted on murals across the country, all with the same line of text “Where is Santiago Maldonado?”

The 28-year-old activist went missing on Aug. 1 during a protest for indigenous rights in Patagonia on the southern tip of South America. Law enforcement officials, who broke up the protest, later claimed Maldonado drowned in a nearby river or escaped over the border to Chile.

Yet the lack of a body or DNA evidence has created a frenzy among Argentinians, who are obsessed with the case, and for good reason given the country’s recent past: They fear the government made him “disappear.”

That is a chilling prospect in a country where as many as 30,000 political opponents of the ruling military dictatorship were grabbed off the streets in the 1970s and ’80s and “disappeared.” They often were interrogated by the military regime, tortured and then dropped into the Atlantic Ocean from an airplane.

That the government is behind Maldonado’s vanishing is “the only hypothesis there is because there aren’t witnesses to support other hypotheses,” said Andrea Antico, Maldonado’s sister-in-law. “We believe that the state is responsible for the forced disappearance of Santiago.”

Antico and other government critics say it is not coincidental that Maldonado’s disappearance comes during the rule of President Mauricio…