International pressure is growing on the Venezuelan government to investigate the deaths of some 35 prison inmates last week, which authorities blamed on drug overdoses. Rights groups, including Human Rights Watch, have called on Caracas to launch a thorough investigation into the deaths as suspicions abound over the official story.   

“Thousands of inmates have died in Venezuelan prisons in recent years, including dozens in the Uribana prison since 2013,” said Jose Miguel Vivanco, Human Rights Watch’s Americas director, in a news release Monday. “How many more need die for the government of Venezuela to effectively address the crisis in its penitentiary system?”

The deaths came amidst an inmate hunger strike over conditions and treatment at the Uribana prison in the northwestern state of Lara on Nov. 24. The authorities initially said 13 prisoners died and 20 remained comatose after a mob raided the medical unit of the prison, ingesting pharmaceutical drugs and alcohol. Days later, officials raised the death toll to 35.

The Venezuela Prison Observatory, a local nongovernmental organization, has questioned the government’s version of events, and cited witness testimonies that the prisoners were poisoned. The observatory estimated that 41 inmates died in the Nov. 24 incident, out of around 200 intoxicated from the drugs. The director of the prison, Julia Cesar Perez, was detained last week over the episode.

The Uribana prison was the scene of a riot last year that resulted in 56 inmate deaths after authorities tried to confiscate illegal firearms from the prisoners.

Last week, the U.N. Committee Against Torture issued a report highlighting “high levels of violence inside prison conditions” in the country, stating that 4,791 people have died in Venezuelan prisons over the past decade, with an additional 9,931 injured. The committee urged the government to allow the special rapporteur on torture to conduct an assessment of prison conditions, citing gaps between reports of prison overcrowding and official statements.