Nicaraguan-American dual national Eddy Montes Praslin was one of 18 imprisoned anti-government protesters killed or wounded on Thursday in a disturbance at the La Modelo prison near the Nicaraguan capital city of Managua.

Montes was reportedly shot dead by prison guards in what U.S. Ambassador Kevin Sullivan called an “unjustifiable use of lethal force against an unarmed political prisoner.”

“We vehemently condemn the killing of Mr. Montes,” the U.S. State Department said on Saturday. “His death, the deaths of hundreds of innocent civilians at the hands of Ortega’s security and parapolice forces, and the continued detention of hundreds more political prisoners demonstrate the regime’s utter disregard for human life and democratic freedoms.”

“We call for an immediate, thorough, and transparent investigation of Montes’s death. We reiterate our call for the unconditional release of all political prisoners. The United States will hold accountable the Ortega regime and Nicaraguan security forces involved in human rights abuses and violations,” the State Department said.

Montes, 56, came to the United States at the age of 13, was educated in California, and served in the U.S. Navy. He moved back to Nicaragua in 2006 and studied medicine and law, obtaining a Nicaraguan law degree in 2018.

Family members said Montes was thrown in jail in October after he complained to police about supporters of Nicaraguan President Daniel Ortega occupying his property. Montes was eventually charged with offenses including terrorism, robbery, and obstruction of public services in what human rights activists described as political persecution.

Montes was a longtime critic of the Ortega government and the Sandinista socialist party, which spent the past year brutally suppressing student demonstrations and rebuffing calls for early elections. At least 325 people have been killed, 2,000 injured, and 52,000 forced to flee Nicaragua by the crackdown.

Activists said Montes brought food, medicine, and other supplies to student protesters. After his arrest and imprisonment, young fellow prisoners began calling him “pastor” and “father” because of his age and his leadership at religious gatherings.

Human rights activists who knew Montes said he claimed to have witnessed a murder by government agents and was prepared to testify against them.

An attorney who represents Nicaraguan political prisoners said over the weekend there is “proof” Montes was shot in the back by prison guards. The official story is that he was among several prisoners who rushed the guards and tried to take their guns during a visit to the facility by the International Red Cross.

Eddy Montes Praslin’s daughter Jafet Montes said Ortega is ultimately responsible for her father’s death.

“I blame the government, I blame the president, because he controls everything that happens in that country,” she said.

She said she was uneasy about traveling to Nicaragua for her father’s funeral because “if they threw my father in jail under any excuse, I feel they could do the same to us.”

The U.S. Embassy in Nicaragua said on Sunday it had received reports of the disappearance of another dissident, student leader Zayda Hernandez. Hernandez was found near the Cathedral of Managua later on Sunday in a state of shock with signs of physical abuse. Opponents of the Ortega regime said Hernandez was abducted and abused when she tried to join a sit-in near the cathedral.