HAVANA (AP) — Non-governmental organizations monitoring Cuban prisons demanded an investigation Tuesday into the death of a man imprisoned since 2021 after being arrested for participating in historic protests against shortages and blackouts.

Authorities say 29-year-old Manuel de Jesús Guillén Esplugas committed suicide in prison. However, relatives claim he succumbed to injuries sustained during a failed escape attempt at Combinado del Este prison in Havana, where he was serving a six-year sentence.

Guillén, a member of an opposition group called the Patriotic Union of Cuba, died on Saturday and his remains were returned to his family over the weekend.

“What we were able to find out, through various sources, is that Manuel was trying to escape from prison, and was caught in the act,” said Camila Rodríguez, representative of Justicia 11J, an NGO that keeps track of the situation of detainees in Cuba. “We will never know for sure what happened, unless they let us enter the prison and interview and reconstruct the events independently,” she added.