The Sydney siege gunman Man Haron Monis attempted to join Australia's Rebels Motorcycle Club but was rejected because the gang considered him irrational and untrustworthy, a former New South Wales detective has revealed.

It is believed Monis, a self-declared Islamic sheikh and holy man, attempted to join the notorious biker gang at some point during the year prior to him holding up a Lindt café where he declared allegiance to the Islamic State.

Dr Michael Kennedy — a former detective in the NSW Organized Crime Squad, Major Crime Squad, Bureau of Crime Intelligence and NSW State Crime Commission — told VICE News he understood that Monis had been rejected because of his behavior.

"He was a nominee for the Rebels last year sometime but he never got to be a member. My information is that they got tired of his antics and became aware that he wasn't to be trusted because he wasn't rational, so asked him to leave," said Kennedy, who now leads the University of Western Sydney criminology program.

Known as "bikie gangs," motorcycle clubs remain a dominant part of the country's criminal underworld. The Australian Crime Commission (ACC) has described them as "the most identifiable components of Australia's criminal landscape."

According to the ACC, members of outlaw bikie gangs "play a prominent role in Australia's domestic production of amphetamine-type stimulants, they are also involved in other illicit drug markets, vehicle rebirthing and firearms trafficking."

"There are gang members involved in all aspects of serious crime," Professor Mark Lauchs, a criminology expert at Queensland University of Technology who specializes in such groups, told VICE News. "They're taken extremely seriously by authorities … in some regions motorcycle gangs are the number one priority as far as organized crime is concerned."

But Lauchs said that Monis would have stood no chance of making it through the entry process for the Rebels.

"To be a nominee you've got to be brought forward by someone and then serve your time — you're literally treated like a slave. Monis might have come forward and asked to join the gang and they said 'no' but I can't see that any chapter would have seriously considered him," said Lauchs.

"He doesn't fit the profile; he styled himself as a cleric so wouldn't be into the partying and as far as I know he didn't ride a motorbike. He clearly didn't know anything about them."

"People join bikie gangs because they're looking for support and brotherhood," continued Lauchs, "so I can see why Monis would have been attracted to that, because he seems to have been a loner. In my opinion Monis would have been attempting to join for those aspects, not for the organized crime."

The Rebels account for 25 percent of all bikie gang members in Australia and law enforcement authorities have routinely pursued them in recent years.

A multi-agency taskforce, codenamed Attero, was set up in 2012 specifically to target the gang. In its first 12 months, the task force arrested 330 people and laid more than 500 charges.

Monis did have an extensive criminal history prior to the siege, and was known to be mentally unstable. He allegedly played a role in the murder of his ex-wife, Noleen Hayson Pal, and was on bail at the time of the siege. He was also facing charges on over 50 counts of sexual assault, beginning in 2002. Between 2007 and 2009 he fought extended legal battles for sending abusive letters to the families of soldiers who had died in Afghanistan.

Monis also wrote rambling communiqués addressed to US President Barack Obama, Australian Prime Minister Tony Abbot and other high-ranking officials.

"The guy had serious mental health issues and he can't be profiled because he was all over the place, behaving differently everyday," said Dr Kennedy. "He wasn't a jihadist, he was just a narcissist after attention. Whether ISIS (the Islamic State) think he did a good thing is irrelevant."

He said the attempt to join the rebels was a sign of Monis's desperation to be part of something, and that he would have adopted any persona that presented him as a radical. "It is even more proof that he was totally and utterly confused and just wanted to be accepted and was happy to settle for anything."

Monis obtained asylum in Australia in 2000 after claiming that he was a high-ranking Iranian spy; he later gained citizenship in 2004.

The Australian Federal Cabinet and New South Wales State Cabinet offices are due to complete a report reviewing Monis, and the history of all government dealings with him, by the end of January. The investigation will report directly to cabinet, and may never be published.

Three people died, including Monis, as a result of the 16-hour siege, during which time he held 17 people hostage inside the Lindt café in Martin Place.

Recent reports have indicated that one of the victims, Katrina Dawson, was killed from a ricocheting police bullet as she protected another hostage, who was pregnant, during the shootout that ended the siege.

An inquest will open on January 29, with New South Wales State Coroner Michael Barnes reporting on how deaths occurred and how they could have been prevented.

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