There was still no confirmation of the identity of the four men, who were arrested on Wednesday.
However, a source close to the investigation said that two arrestees were Zeljko Maglov, a former commander of the 73rd Military Police Battalion, and Tvrtko Pasalic, a former head of the now defunct Security and Intelligence Service in Sibenik.
According to media speculations, the other two arrestees are a former commander of the Kuline military prison, Damir Borsic, and Milorad Paic, a former military policeman.
In 1993, the Defence Ministry and intelligence services had information about wrongdoings in the prison and they set up a commission to establish what exactly had happened there.
According to a report issued by the Defence Ministry in 1999, the commission established that some commanders had demonstrated self-will in carrying out orders, and consequently two military police officers -- Milorad Paic and Mario Barisic -- were suspended.
The Defence Ministry closed the case as no other criminal activities had been detected in that inquiry.
Zeljko Maglov came into the media limelight as a witness in the Lora war crimes trial and in some other trials for car and sugar smuggling. He was also arrested for drug possession.
Upon the four men's arrest on Wednesday, the Ministry of the Interior said in a statement that the investigation was still in progress. It did not reveal the identity or age of the suspects.