Four inmates were killed and seven others wounded when a grenade went off inside Injuba prison in the northwestern state of Barinas, Venezuela's official AVN news agency said Monday.

"Authorities are reviewing the reports to determine what were the causes that generated the event," state government secretary Enrique Henriquez told AVN.

Humberto Prado, director of the independent Venezuelan Prisons Observatory, told Efe the blast occurred Sunday night as a group of inmates "played" with a grenade.

The grenade incident was apparently connected with the murder hours earlier of a 50-year-old inmate shot five times by fellow prisoners, Prado said.

Three-quarters of the 1,700 inmates at Injuba are still awaiting trial, according to the Observatory, which documented 476 killings inside Venezuela's badly overcrowded prisons in 2010.