Iranian refugee Man Haron Monis, who has been reported as either 49 or 50 years old, moved to Australia in 1996 where he assumed the title of “Sheikh Haron”.
Well-known to the authorities since he was found to have written hate letters to the families of Australian soldiers killed in overseas conflicts, Monis operates an extremist website, YouTube channel, Facebook and Twitter pages in which he describes his various police charges as part of a witch hunt against him.
He was also banned in 2010 from writing similar "letters of condolence" to British soldiers killed in that conflict. He was charged with being an accessory to the murder of his ex-wife and mother of his two children last year, The Age reported.
It is believed Monis is out on bail after he was arrested earlier this year over sexual assault allegations stemming from 2002.
Police in that case alleged he put adverts in his local paper claiming to be a spiritual healer with expertise in astrology, numerology, meditation and black magic, and indecently and sexually assaulted a woman who visited him.
It is alleged he told the woman the attacks were part of a spiritual healing technique, and he warned the alleged victim not to tell anyone about them.
"There's no operational reason for that name to be held back by us now," said a police source, who declined to be identified.
On his website, Haron posted an update yesterday that read:
"Islam is the religion of peace, that's why Muslims fight against the oppression and terrorism of USA and its allies including UK and Australia. If we stay silent towards the criminals we cannot have a peaceful society. The more you fight with crime, the more peaceful you are. Islam wants peace on the Earth, that's why Muslims want to stop terrorism of America and its allies. When you speak out against crime you have taken one step towards peace."
In one video posted on his YouTube channel, Haron can be seen holding a placard that says "I have been tortured in prison for my political letters".
On his Twitter account, Haron has posted numerous messages in support of "Team Islam" and criticising "racist and terrorist Australians".
Around 15 hostages were seized after the gunman entered the Lindt chocolate cafe on Martin Place in the city's busy financial and shopping district on Monday carrying a black Islamic flag at around 9.45am local time.
Five hostages have escaped the siege and hundreds of police remain encamped around the shop in Martin Place. The gunman has reportedly told police that he has planted four bombs around the city.
As the siege continued and evening descended, one employee was seen turning the lights of the cafe off - plunging the shop into complete darkness. Hundreds of police and emergency services remain in the area and negotiators have been in direct contact with the gunman.
Earlier, dramatic footage showed five people - three men and two women - managing to flee the scene.
It is understood the five, some of whom were wearing brown aprons with the Lindt logo on them, escaped rather than being released by the gunman. One woman who escaped has been widely named as Elly Chen.
As the siege unfolded, two people inside the cafe were seen holding up a flag with an Islamic declaration of faith that has often been used by extremists, fuelling speculation that it was the work of an Islamic extremist. He has since reportedly requested an Isis flag, with some outlets reporting that he took the wrong flag into the cafe.
It was not clear exactly how many people remain inside the cafe although staff at Channel 7, a TV news channel which has offices directly opposite the cafe, counted about 15 different faces among hostages forced up against the windows.
Chris Reason, a reporter at Channel 7, said the man carried what appeared to be a pump-action shotgun, was unshaven and wore a white shirt and a black cap.
Mr Reason also reported that the lights in the cafe have now been turned off, adding that the black flag can still be seen being held up against the window.
He claimed the gunman could be seen "getting extremely agitated" and was shouting at the remaining hostages when the five people escaped earlier today.
Police have confirmed that negotiators have made contact with the gunman, who is also thought to be middle-aged.
Sky News has also reported that the gunman used some of the hostages as human shields in the window.
He is understood to have have made known a list of his demands to local news organisation through phone calls made from inside the cafe.
New South Wales Police has requested that media organisations do not repeat the gunman's demands or relay what the hostages have said during these calls.
Australia's Channel Ten has reported that he told police that he had placed two bombs in the city's central business district and had a further two in the cafe with him.
Channel Ten also claims to have received a video in which a hostage has relayed the gunman's demands, according to its anchor Hugh Riminton, who said that the video will not be broadcast in accordance with police requests.
Police are not providing details on the operation and whether it was the force who have turned off the lights or the people inside the building, and have not confirmed whether the five people who escaped the building did so by their own means or were set free by the gunman.
Police negotiators have revealed they suspect they now know the name of the gunman, but have asked media not to reveal his identity.
New South Wales's Police Commissioner Andrew Scipione said at a press conference: "I understand that no one has been injured, and if that is true then we are grateful."
"Our only goal is to get those people who are currently caught in that building out of there safely. Nothing will change."
Footage broadcast earlier on Sky News showed two men running towards a group of police around what appeared to be a fire escape door. In a separate shot another man, wearing a brown apron, was seen running out through the door.
Australia is on high alert for attacks by radicalised Muslims or by home-grown militants returning from fighting in the Middle East, having raised its threat level to high and undertaken a series of high-profile raids in major cities.
Prime Minister Tony Abbott has said that at least 70 Australians were fighting in Iraq and Syria backed by about 100 Australia-based "facilitators".