Brooklyn Rivera, an Indigenous leader, politician and activist, has died at age 73 after years in Nicaraguan state custody, prompting outcry from rights advocates.

On Sunday, Nicaragua’s government attributed his cause of death to a bacterial infection that took hold after a bout of COVID-19.

But critics have expressed scepticism and outrage, as the announcement came after growing pressure to ascertain his welfare.

“If he is dead, it cannot be said that the cause was illness,” said Reed Brody, a member of the United Nations Group of Human Rights Experts on Nicaragua.

In a statement before Rivera’s death was confirmed, Brody blamed the government for any harm to the Indigenous leader.

“The cause would be that he was in government custody in conditions of enforced disappearance for over two years, denied independent medical oversight. There is no other way to read this,” Brody wrote.

Since September 2023, Rivera has been held in state detention, without contact with the outside world. Until recently, there had been no confirmation of his imprisonment, and his family was barred from seeing him.

But on Wednesday, the Ministry of the Interior confirmed Rivera’s detention and published photos of the Indigenous leader intubated in a hospital.

It described Rivera’s condition at the time as “delicate”. He had reportedly suffered from “multiple organ failure, a cirrhotic liver and an active lung infection”, and he was being treated with “mechanical ventilation through a tracheotomy and intravenous feeding”.

The photographs spurred a new wave of condemnation and calls for his freedom.

The United States “demanded his unconditional release” in a statement posted to social media. It also blamed Nicaragua’s leaders for “their singular role in his cruel treatment”.

“This repression, violence, and inhumanity is abhorrent; we reiterate our call for his and all political prisoners’ unconditional release NOW,” the US State Department wrote.

Nicaragua’s government – led by spouses Daniel Ortega and Rosario Murillo, who serve as co-presidents – has long been criticised for its hardline rule and record of human rights abuses.

Under Ortega and Murillo, dissidents have faced arrest, imprisonment, torture, exile and the revocation of their citizenship.

Rivera was among the leaders who spoke out against Ortega’s left-wing Sandinista government.

A member of the Miskito Indigenous group, Rivera has advocated for the protection of his people’s ancestral lands, along Nicaragua’s northeast coast.

The territory has faced pressure from government and business interests seeking to exploit its rich deposits of gold, silver and other resources.

Rivera was also involved in the fight against the country’s first Sandinista government, from 1979 to 1990, as the leader of the Misurasata armed group.

In 1980, he went into temporary exile in neighbouring Costa Rica. A Sandinista attack after his return forced him once again to seek safety abroad, this time in Colombia.

Rivera would go on to co-found Yamata, an Indigenous political party that helped secure limited autonomy for Indigenous peoples following peace negotiations with the Sandinistas.

Ortega eventually returned to power in 2007. In recent years, he has passed reforms to consolidate his control over the government, including by elevating his wife, Murillo, from vice president to president.

In his last years of freedom, Rivera continued to speak out against the government.

In April 2023, he travelled to Geneva, Switzerland, to address a UN forum on Indigenous peoples. After delivering remarks critical of Nicaragua, he was banned from re-entering the country.

Rivera nevertheless smuggled himself back into the country and was living in hiding until his arrest in September 2023. The government charged him with alleged terrorism, but critics said his arrest amounted to the silencing of the Indigenous leader.

“Nobody heard from him since then,” Brody said. “The government never gave any indication. He was a disappeared person.”