Video report by ITV News Security Editor Rohit Kachroo
Two men tried to smuggle an improvised explosive device on to a plane at an Australian airport last month in a plot directed by so-called Islamic State, police have said.
The pair - who are now facing terrorism charges - targeted an Etihad Airways flight out of Sydney airport, Australian Federal Police Deputy Commissioner Michael Phelan said, calling the plot "one of the most sophisticated... attempted on Australian soil".
One of the men, a 49-year-old, brought the device to the airport on July 15 in a piece of luggage that he had asked his brother to take with him on the flight - but he had not told his brother the bag contained explosives, the deputy commissioner added.
He continued that for reasons that are still unclear, the bag never got past the check-in counter.
Instead, Mr Phelan said, the 49-year-old man left the airport with the bag, and his brother continued on to the flight without it.
The man's brother has not been charged in connection with the plot, because police believe he did not know that the bag contained explosives.
Friday's comments are the first made by officials since four men were arrested in the Surry Hills area of the city in a series of raids.
A 49-year-old man, from Sydney, and a 32-year-old have been charged with two counts of planning a terrorist act. A third man remains in custody, while a fourth was released without charge.
Police have not released any names in connection with the investigation.
The components for the device, including what Mr Phelan described as a "military-grade explosive," were sent by a senior so-called Islamic State member to the men in Sydney via air cargo from Turkey, he said.
A so-called Islamic State commander then instructed the two men who have been charged on how to assemble the device, which police have since recovered, Mr Phelan continued.
After the July 15 bid failed, the men changed tactics and were in the early stages of devising a chemical dispersion device, which they hoped could release highly toxic hydrogen sulphide, he said.
No specific targets had been chosen, though a so-called Islamic State member overseas had given the men suggestions about where such devices could be placed, such as crowded areas or on public transport.
"Hydrogen sulphide is very difficult to make, so I want to make it quite clear that while it may be a hypothetical plot, we were a long way from having a functional device," Mr Phelan said.
"There were precursor chemicals that had been produced, but we were a long way from having a functioning (device)."
Mr Phelan said police had no idea either of the plans were in the works until they received a tip-off through intelligence agencies on July 26.
The allegation that so-called Islamic State was able to ship explosives to Australia undetected was troubling, police admitted.
"All the security agencies and those responsible for security of cargo and so on have put in place extra measures since that time," Mr Phelan said.
"It is concerning that it got through, yes, it's hard to deny that."
After learning of the plot, Mr Phelan said police made a similar mock IED and ran it through the airport's luggage system, and it was detected by security.
One of the men charged was put in touch with the so-called Islamic State commander police believe directed the plot in April, Mr Phelan said.
The two men who have been charged are scheduled to appear in court on Friday. If convicted, they could face a sentence of life in prison.