Sydney's CBD was brought to a standstill as an armed man, identified by police as Man Haron Monis, held 17 hostages in a Martin Place cafe. Little is certain about what happened inside the cafe before police stormed the building but at one point hostages held a black flag with Arabic writing on it up to a window.
The flag appears to be a variation of what is known as the Black Standard, an Islamic flag now widely used by Islamic extremists and militant jihadists.
The flag's appearance immediately drew comparisons with the Islamic State terrorist group, whose blitz through Iraq and Syria has been heralded by similar-looking black flags.
However, the flag displayed above hostages at the Lindt Chocolate Cafe in Sydney is different from the one Islamic State uses.
It is also worth putting what we know about the flag into context. As Lowy Institute military fellow James Brown has pointed out: "All we know for sure at the moment is that a crime is being committed and someone involved wants to suggest an Islamic association."
What are the flag's origins?
The Black Standard flag dates back to the early days of Islam.
Since this time, black flags have been used by a number of Islamic organisations and governments.
The flag pressed against the glass in the Sydney siege consists of a black background with an Islamic declaration of faith printed on it in white text.
The creed printed on the flag is known as the Shahada and it translates as: "There is no God but Allah and Mohammed is the messenger of God."
It is an extremely common statement of faith for Muslims.
Is it associated with extremism?
Shahada is a central tenet of Islam; the text of the creed itself is not directly linked to extremism or jihad.
It is commonly found on numerous Islamic flags, including that of countries such as Saudi Arabia.
However, variations of the black flag with the Shahada printed on them in white text have in recent decades become commonly used by extremist Islamic groups.
Jabhat al-Nusra, commonly known as Al Qaeda in Iraq, uses a flag with the Shahada written on it, followed by the group's name written on an additional line.
A black flag with the Shahada written on it is used by the organisation Hizb ut-Tahrir, which campaigns for a worldwide caliphate, ruled by Islamic law.
Hizb ut-Tahrir is not currently banned in Australia but has been accused of having links to terrorist organisations.
Is it the flag of IS? No
The Islamic State terrorist organisation's flag is a variation of the Shahada on a black background, however the second paragraph, "Mohammed is the messenger of Allah" is depicted in the form of the historical Seal of Mohammed.
So the IS flag is similar in substance to the flag used during the siege, but IS has not used that exact flag.