A WEEK before Christmas, the world held its breath as a tense hostage situation unfolded in a cafe in Sydney's central business district.
A black flag bearing the Islamic declaration of faith was pressed against the cafe window by hostages, sparking a wave of speculation that the Islamic State in Iraq and Syria (ISIS) was behind the attack.
But the lone gunman - who died with two of his hostages at the end of the 16-hour siege - had no direct links to a terror group.
Man Haron Monis, a self-declared sheikh with a rapsheet that included murder, was a "lone wolf" who, though jolted into action by a radical and violent ideology, acted on his own.
The Sydney siege has thrown the spotlight on the lone-wolf terrorist and raises questions about how ready Singapore is to face such an attack on its shores.
There have been at most 120 true lone wolves in the last three decades of terrorism, says S. Rajaratnam School of International Studies (RSIS) associate professor Ahmed Salah Hashim.
He explains: "A true lone wolf is a self-radicalised individual with a political agenda but no direct links to any terrorist group. He is not part of a sleeper cell. He works alone, even if he is influenced by the ideas of a group."
The term "lone wolf" first gained traction in the 1990s, when white supremacists urged people to commit crimes alone or in small groups.
When Anders Breivik launched a spate of attacks in Norway that killed 77 people in 2011, Singapore's Deputy Prime Minister, Mr Teo Chee Hean, warned then that the lack of formal links to established terror organisations makes such lone-wolf terrorists difficult to detect and monitor.
Four years on, these concerns have resurfaced. While experts Insight spoke to this week say that lone wolves have not completely supplanted terrorist groups, they warn that these individuals - working in quiet isolation - pose a serious threat because identifying them is a challenge.
They are coming about as terror groups find themselves battered by heightened scrutiny, says Dr Bilveer Singh, who has studied terrorism in South-east Asia for the past 30 years. "As terror groups have been targeted, it is a liability to be part of these groups," he explains.
A complex variety of motivations make these lone wolves tick. For some, it is their flawed understanding of religion; others are fuelled by social and political grievances.
Unabomber Theodore Kaczynski, for example, was an anti-industrialist. Meanwhile, Breivik claimed he killed out of "goodness, not evil".
Due to their scattered motivations, the lone wolves, experts agree, are particularly dangerous because they are almost impossible to detect.
Criminology professor Mark Hamm from Indiana State University, who has studied 98 cases of lone-wolf attacks in the United States, points out that the Internet - which has also been blamed for radicalising individuals - can also help weed out those plotting attacks of their own.
Lone wolves are likely to signal their intent online, he adds, via statements supporting radical ideology to their friends or family on social media, for instance.
Terror groups like Al-Qaeda and ISIS, meanwhile, are renewing calls for more lone-wolf attacks. And though classical terrorist organisations - hierarchical structures with networks that span the globe - will remain, lone wolves will be a force to be reckoned with in the years to come, say experts.
"Small groups and resilient networks have become new organisational paradigms, especially in the case of jihadist terrorism. Lone wolves are the product of this evolution and represent a sustainable trend," explains Mr Romain Quivooij, an associate research fellow who studies radicalisation at the Centre of Excellence for National Security at RSIS.
And these individuals will have a devastating effect on a society's psyche, he adds. "The message conveyed by lone wolves, which can be expressed by 'We are among you and we will strike', is a strategic challenge."