Police stepped up their investigation into the death of Yonatan Alzam over the weekend after concluding that the prisoner - who was supposed to testify Tuesday in a high-profile murder case - was murdered in his prison cell. Alzam, a 22-year-old serving a life sentence for his part in an underworld killing, passed away under mysterious circumstances early Tuesday morning only hours before he was supposed to take the stand as the primary witness in a high-profile underworld murder case. An autopsy conducted at the National Forensic Institute on Thursday found a chemical substance in Alzam's body, leading police to believe that his death hours before he was supposed to incriminate a main underworld figure was not natural. On Friday, Cmdr. Dudi Cohen - head of Police Investigations and Interrogations - convened senior investigators from the Central District and decided to transfer the investigation from their hands to the police's elite Serious and International Crimes Unit (SICU). "Everything points to murder," police said over the weekend explaining why the investigation was transferred to the SICU which usually runs investigations with an overseas connection. "This is a complicated and sensitive case since Alzam was murdered in his cell in a high-security prison where he was supposed to be under 24-hour protection." Police added that the investigation demanded extreme caution. "Prison guards might be involved," they warned. Alzam was sentenced last month to life in prison for his part in the 2003 murder of underworld crime figure Hanania Ohana in a Petah Tikvah parking lot. Alzam was arrested together with three other suspects. On Tuesday he was scheduled to testify against known criminal Shimon Zirhan who allegedly pulled the trigger and shot Ohana in front of his wife and child. On Friday, police began investigating wardens and prisoners at Ashmoret prison. No signs of violence were found on Alzam's body, but an acrid-smelling chemical substance was found in his cell, strengthening police suspicions that the prisoner was poisoned.