Police release one in terror investigation as it's revealed 'Mother of Satan' gas may have been part of alleged plot

One of four men arrested in Sydney for an alleged terror plot to bring down a plane has been released.

The Australian Federal Police advised on Wednesday morning that a 50-year-old man, Abdul Merhi, was released on Tuesday night without being charged.

Mahmoud Khayat, Khaled Khayat and Khaled Merhi remain in police custody as counter-terrorism police continue to investigate an alleged plot to bring down a plane after raids on four Sydney properies on Saturday.

The men are being held under 'specified time' provisions outlined in Commonwealth Crimes Act.

Alleged plot may have involved 'Mother of Satan' gas

A terror expert believes the alleged terror plot may have centred around using a gas referred to as the “Mother of Satan”.

Deacon University terror expert Greg Barton suggests the alleged plan may have centred around acetone peroxide, or TATP, the same chemical used in the Manchester Arena bombing in May and the 2015 Paris attacks.

Referred to as the "Mother of Satan" because it can kill those handling it, TATP is unstable but powerful and would not give off "tell-tale" chemicals picked up by airport swab tests, Prof Barton said.

"That makes it a candidate for using in this sort of attack."

An alleged Sydney terror plot may have centred around the same gas used in the Ariana Grande concert attack in Manchester in May. Source: Getty

Concert-goers flee Manchester Arena in fear after bomb goes off. Source: Facebook

TATP, which needs a pressure vessel to be packed in, could be placed in a grinder so it was opaque through an X-ray machine and appeared innocuous upon visual inspection.

"It might just pass (security)," Prof Barton said.

"That, I'm guessing, was their plan," he alleged.

Prof Barton speculated while the alleged plot could have succeeded it more likely would have failed because the bomb would have been detected, failed to detonate or not have detonated to its full potential.

Khaled Khayat, one of three men still under investigation. Source: 7 News

A Police officer searches items from a property in relation to counter-terrorism raids in Lakemba. Source: AAP

"(Still) it's important to understand this clearly is the most sophisticated [alleged] terror plot we've had, probably ever, in Australia," he added.

News agency Reuters on Tuesday cited two US officials familiar with the arrests as also saying the Australian investigation wasn't a sting operation but the result of the detection of a developing plot.

On Saturday night, four men were arrested in counter-terror raids conducted in four suburbs across Sydney. Photo: AAP


Etihad helping with investigation

The alleged terror plot has been linked to an Etihad Airways flight from Sydney to Abu Dhabi. Etihad has confirmed it's helping authorities.

"The Etihad Airways aviation security team is assisting the AFP with its investigation and the matter is ongoing," the airline said in a statement.

"Etihad is complying fully with the enhanced security measures at airports in Australia and monitoring the situation closely."

Investigators on Monday found a Qantas release slip relating to a Sydney to Jakarta flight near one of the raided Sydney properties but it is understood the plot did not involve Qantas or the Qantas Group.

Etihad are assisting with investigations. Source: Getty Images

Authorities have refused to officially confirm any details about what they've found after rifling through properties in four Sydney suburbs.

US officials, speaking to Reuters on condition of anonymity, said communications between the alleged plotters in Sydney and members of Islamic State in Syria were intercepted by a foreign intelligence service.

The US officials declined to identify the service and United Kingdom officials refused to confirm or deny playing a role in detecting the plot.

Police carry items from the scene of a counter-terrorism raid in Lakemba. Source: Getty Images

Australian National University intelligence expert Professor John Blaxland says relying on international tip-offs shouldn't raise questions about the adequacy of Australia's intelligence services.

"What we know is that organisations like the AFP and ASIO have extensive networks of connections and liaisons arrangements with counterparts globally," Prof Blaxland told said.

"There's a mutual benefit - a mutual back-scratching exercise.

"That network is proving more and more useful and more and more important for countering potential terrorist acts."

Police and investigators searching properties. Source: 7 News