Police disrupted the first alleged plot in Australia to bring down an airplane and arrested four men in raids on Sydney homes, officials said Sunday.
Prime Minister Malcolm Turnbull said that security has been increased at Sydney Airport since Thursday because of the plot. The increased security measures also were extended to all major international and domestic terminals around Australia overnight.
"I can report last night that there has been a major joint counterterrorism operation to disrupt a terrorist plot to bring down an airplane," Turnbull told reporters. "The operation is continuing."
Australian Federal Police Commissioner Andrew Colvin said details were scant on the specifics of the attack, the location and timing.
"In recent days, law enforcement has been become aware of information that suggested some people in Sydney were planning to commit a terrorist attack using an improvised devise," Colvin said.
Deakin University security expert Greg Barton said the first plot to target aircraft in Australia, which is the highest aspiration of many extremists, was a "pretty big threshold moment."
The plotters were apparently making a peroxide-based explosive device rather than using nitrate-based chemicals that can be detected by airport security swab tests, Barton said.
Manchester Arena bomber Salman Abedi used such a peroxide-based explosive, triacetone triperoxide, better known as TATP, to kill 22 concert-goers in Britain on May 22.
"TATP's called Mother of Satan because it often kills the bomb maker because it's very unstable as it's mixed," Barton said. "But if it's mixed well, it can be very potent and a small amount can be enough to bring an aircraft down if it's done very, very expertly."
The plan most likely was to take the explosive on board in carry-on luggage unless there was a baggage handler involved who could ensure that a stowed bomb exploded near the fuselage where it would be most damaging.
"The speculation is that the bombers would like to put it in carry-on luggage so they can be sure of getting it placed near the fuselage skin," Barton said, adding however that putting something in a suitcase is "a lottery whether it ends up near the outside of the luggage hold or packed near the middle."
There was no evidence that airport security had been compromised, Colvin said.
"We believe it's Islamic-inspired terrorism," Colvin said when asked if the Islamic State group was behind the plot.
Seven Network television reported that 40 riot squad officers wearing gas masks stormed an inner-Sydney house before an explosives team found a suspicious device. Colvin declined to say whether a fully equipped improvised explosive device had been found at that address.
A woman led from a raid by police with her head covered told Nine Network Television: "I love Australia."
None of the four suspects arrested in five raids had been charged, Colvin said. He would not discuss what charges they might face. None of the arrested men worked in the airport industry, Colvin said.
Australia's terrorist threat level remained unchanged at "probable," Turnbull said. He advised travelers in Australia to arrive at airports earlier than usual — two hours before departure — to allow for extra security screening and to minimize baggage.
The plot was the 13th significant threat disrupted by police since Australia's terrorist threat level was elevated in 2014, Justice Minister Michael Keenan said. Five plots have been executed.
Since Australia's terrorist threat level was raised in 2014, 70 suspects have been charged in 31 police operations, Keenan said.