"We are not like other countries ... we are more threatened and face more challenges" ... Israeli Prime Minister Benjamin Netanyahu, centre. Credit:AFP

In his first public comment on the death of Mr Zygier, Israel's Prime Minister Benjamin Netanyahu defended the country's security services and warned that "the over-exposure of security and intelligence activity could harm, sometimes severely, state security".

"We are not like other countries ... we are more threatened and face more challenges; therefore, we must maintain proper activity of our security agencies," he said after the government's weekly cabinet meeting on Sunday, describing the security forces as operating with "endless dedication and commitment" to Israel's safety.

As is the norm in Israel's criminal justice system, Mr Zygier, did not have the support of his legal team when he was being questioned over what are believed to be treason charges.

The State Attorney's office, located in the country's Ministry of Justice, has been charged with conducting the investigation into whether negligence on the part of Israel's prison authorities contributed to Mr Zygier's death. That means the organisation responsible for deciding to arrest and indict the Australian and who oversaw his secret trial and the conditions of his incarceration will decide whether his suicide in custody - purportedly under the 24-hour surveillance of prison service guards - was a result of negligence. A senior Justice Ministry official - speaking on condition of anonymity - told Fairfax Media there was no conflict of interest in the investigation, saying that if negligence led to Mr Zygier's death, people would be held responsible. ''The Ministry of Justice was not guarding him, this is the office of internal security - these are two different offices,'' he said.