Police received the initial findings from the autopsy of Yonatan Alzam on Thursday morning; Alzam died mysteriously in his prison cell the night before he was to testify in the murder trial of Shimon Zrihan.
The preliminary results stated that Alzam died an "unnatural death," with police leaning toward poison as the specific cause of death.
However, they have yet to officially rule out a natural cause of death.
Chief Superintendent Micha Levy from the Central Investigative Unit in the Central District is leading the police team set up to probe the death.
The High Court of Justice on Wednesday gave police the go-ahead to conduct an autopsy on the body of Alzam.
Alzam's father and brothers petitioned the High Court against a decision by the Netanya Magistrate's Court to allow the autopsy, despite its opposition allegedly for family and religious reasons.
"The preliminary investigation showed that despite the strict conditions of isolation, there were indications that the guidelines for isolating prisoners were not scrupulously observed," wrote presiding Justice Dorit Beinisch. "There is a real reason to suspect that [Alzam's] death was the result of a criminal act based on the fact that someone might have wanted to prevent him from testifying, especially given the type of crime he was to testify about, and the items found in his cell."
During the hearing, the state's representative, attorney Avi Licht, told the court that authorities had found a bottle of liquid with a sharp smell and cups of coffee in Alzam's cell. These items should not have been available to someone being held in isolation and aroused suspicions he might have been murdered, said Licht.
Licht added that finding the cause of Alzam's death could be of crucial importance in Zrihan's trial.
Attorney David Schussheim, who represented Alzam's family, said there were no external signs to indicate that the prisoner had been murdered. He added that Alzam had been under severe stress over the fact that he was to testify against Zrihan, his alleged accomplice in the killing of Hanania Ohana in 2003.
Schussheim suggested that the 22-year-old Alzam may have died of a heart attack.
That hypothesis may have received support on Wednesday night when police confirmed a Channel 2 television report to the effect that underworld figure Yehiye Turk, who was held in the cell next to Alzam, said Alzam had told him he was under extreme pressure. But the police also said that Alzam's family had not visited him in prison and that before his death, his visitors were mainly members of the Abergil crime organization, which had allegedly hired Zrihan and Alzam to murder Ohana.