Police describe 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.
A senior ISIS commander sent parts -- including weapons-grade explosives -- by air cargo from Turkey intending to build an improvised explosive device, Australian Federal Police Deputy Commissioner National Security Michael Phelan said during a press conference Friday.
The other scheme involved a plan to release a toxic gas in public that was foiled when the accused couldn't produce the deadly gas.
Two men living in Sydney, identified by CNN affiliate Seven News as Khaled Khayat, 49, and Mahmoud Khayat, 32, were charged with terror-related offenses Thursday.
The two appeared by video link Friday in Sydney's Parramatta Court. No plea has been entered. Neither man applied for bail, and a 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, and a fourth has been released.
After the foiled plan to down the plane was revealed Saturday, authorities described it as an Islamist-inspired plot, but they did not link it to a specific terror group until Friday.
Four arrested in alleged airplane terror plot 01:39
Brother to be unwitting bomber
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," the Australian Federal Police's Phelan said.
One of the suspects planned to plant the IED on his brother, who was unaware of his role in the planned attack, Phelan said. The brother is currently abroad, and there are no plans to arrest him
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 the device was in luggage due to be checked in, rather than carry-on baggage.
Seven News reported that police had found parts of a meat grinder at the suspects' home, which they suspect was to be used to carry the explosives aboard the plane.
Phelan 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 accused men received the bomb parts in Australia and assembled what police believe was a "full functioning" IED, he said.
The Australian Federal Police's Michael Phelan says suspects assembled a functioning explosive device.
Toxic chemical
The second terror plot in which 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 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" the device would be used on an airplane, Phelan said.
Hydrogen sulfide is highly toxic, and it has a particular smell, Ian Musgrave, a molecular pharmacologist and toxicologist at the University of Adelaide, told CNN. When inhaled, the gas can cause respiratory paralysis and death. It can be made with high-school laboratory equipment, but a large amount of the compound is needed to be effective.
Concentrations of more than 500 parts per million (ppm) of hydrogen sulfide can result in asphyxia, Musgrave said. Concentrations of 700 ppm will result in death if not rescued promptly, he said.
However, no evidence exists that the device was completed due to the difficulty of producing the highly toxic chemical, he said.
"We were a long way away from having a functional device," Phelan said.
Ongoing investigation
Two search warrants of properties in connection with the case are ongoing, he said.
Authorities have carried out raids at properties across Sydney since Saturday, including the suburbs of Surry Hills, Lakemba, Wiley Park and Punchbowl.
Police prepare to search for evidence Monday in the Sydney suburb of Lakemba.
Investigators were seen rifling through garbage and removing items from houses, dressed in full protective gear.
Threat level lowered
On Thursday, Prime Minister Malcolm 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 Saturday's arrests, 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 a representative for the Australian Security Intelligence Organization.
As many as 70 people have been charged as a result of 31 counterterrorism operations, the representative said.
Correction: An earlier version of this story stated the two men charged had pleaded not guilty. No plea was entered.
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