Twin terror plots, one involving the bombing of a passenger plane and the other a potential poison gas attack, have been described by police as the "most sophisticated" ever attempted on Australian soil.
A senior ISIS commander sent parts -- including weapons-grade explosives -- by air cargo from Turkey with the express aim of constructing an improvised explosive device (IED), said Australian Federal Police Deputy Commissioner National Security Michael Phelan during a press conference Friday.
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 Thursday.
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 CNN affiliate Sky News Australia. "Once the brief of evidence is served, we can assess their legal position."
One other man remains in police custody.
After the foiled plan to down the plane was revealed Saturday, authorities described it as an Islamist-inspired plot, but did not link the plan to a specific terrorist group until Friday.
The would-be attackers planned to place the IED on an Etihad Airways flight on July 15 but "at no stage did the IED breach airline security," Phelan said.
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.
"We were a long way away from having a functional device," Phelan said.
Two search warrants of properties in connection to the case are ongoing, he said. One man remains in police custody, and a fourth has been released.
Multiple raids have been carried out at multiple properties across Sydney by law enforcement authorities since Saturday, including the suburbs of Surry Hills, Lakemba, Wiley Park and Punchbowl.
Investigators were seen rifling through garbage and removing items from houses, dressed in full protective gear.
Threat level lowered
On Thursday, Turnbull said the threat to aviation in Australia had been "disrupted and contained" following the arrests and the level of security at airports was being lowered.
Stricter airport security measures had been put in place following the attack, with Australian airports warning of possible delays and longer check-in times.
There have been five attacks, and 13 "major counterterrorism disruptions" -- including Saturday's arrests -- in Australia since the national terrorism threat level was raised in September 2014, according to an Australian Security Intelligence Organization (ASIO) spokesperson.
As many as 70 people have been charged as a result of 31 counter terrorism operations, the spokesperson added.
"Since 2012, around 220 Australians have traveled to Syria or Iraq to fight with or support groups involved in the conflict. Around 40 people have returned to Australia. Some of these returnees remain of significant security concern.
"Around 110 Australians are currently fighting or engaged with terrorist groups in Syria and Iraq. At least 65, and possibly as many as 77, Australians have been killed as a result of their involvement in the conflict."