police has described the recent twin terror plots, one involving the bombing of a passenger plane and the other a potential poison gas attack, as the “most sophisticated” ever attempted on Australian soil.

Australian Federal Police Deputy Commissioner National Security Michael Phelan revealed durng a press confrence, that the first is supposed to have been sent b a senior ISIS commander via air cargo from Turkey with the express aim of constructing an improvised explosive device (IED). The cargo included –weapons-grade explosives.
The other scheme involved a plan to release a toxic gas in public, but was foiled due to the accused being unable to produce the deadly gas.
Two men living in Sydney, identified by CNN affiliate Seven News as Khaled Khayat, 49, from Lambeka, and Mahmoud Khayat, 32, from Punchbowl, were charged with terror-related offenses.
The two appeared by video link in Sydney’s Parramatta Court Friday and pleaded not guilty to the charges. Neither man applied for bail and the court hearing has been deferred until November 14, after a brief of evidence was requested.
“At the moment, all I can say is they are entitled to the presumption of innocence,” their legal representative, Michael Coroneos told Sky News Australia. “Once the brief of evidence is served, we can assess their legal position.”
However a third suspect man remains in police custody. Although the  foiled plan to crash the plane was revealed on Saturday, and authorities did described it as an Islamist-inspired plot, but did not link the plan to a specific terrorist group until Friday.
According to Phelan, the device didn’t get past the airline’s check-in desk, and a subsequent test of airport security using a dummy device was performed, resulting in the decoy also being found. Phelan said that the device was in luggage that was due to be checked in, rather than carry-on.
He did not elaborate on why the attack did not proceed as planned, beyond saying there was “a little bit of conjecture as to why it did not go ahead.”
The person due to have carried the IED onto the plane was the brother of the older of the two accused men. Phelan said he was unaware of his role in the alleged attack. He is currently abroad and there are no plans to arrest him.
The accused men received the bomb parts in Australia and assembled what police believe was a “full functioning” IED, he said.
The second terror plot, the two men have been charged in connection with, involved an attempt to create a “improvised chemical dispersion device” to release hydrogen sulfide, Phelan said.
It is suspected that the device would have been used to disperse the toxic chemical in “closed spaces, potentially public transport.”
However, there is “no information at all to suggest that” the chemical dispersion device would be used on an airplane, he said.
Hydrogen sulfide is highly toxic and it has a very particular smell, Ian Musgrave, a molecular pharmacologist and toxicologist at the University of Adelaide told CNN. The gas, when inhaled, can cause respiratory paralysis and death. While the gas can be made with high-school laboratory equipment, a large amount of the compound is needed to be effective.
Concentrations of over 500 parts per million (ppm) of hydrogen sulfide can result in asphyxia, Musgrave says. Concentrations of 700 ppm will result in death if not rescued promptly, he added.
However, due to the difficulty of producing the highly toxic chemical, there is no evidence that the device was completed, he said.