IS-K is an abbreviation of Islamic State-Khorasan – a regional affiliate of the Islamic State group, which has been proscribed as a terror organisation by governments across the world.

It is focused on Afghanistan, Iran and Pakistan and into Central Asia.

The group has given itself the name Khorasan as that was part of an historic Islamic caliphate spanning that region.

IS-K has been around for nine years but in recent months it has emerged as the most dangerous branch of the Islamic State group, with a long reach and a reputation for extreme brutality and cruelty.

Along with what is left of the group’s wider leadership in Syria and Iraq, IS-K aspires to a pan-national Islamic caliphate ruled through an ultra-strict interpretation of Sharia, Islamic law.

In Afghanistan it is waging a sporadic but still deadly insurgency against the country’s rulers, the Taliban, who it opposes on ideological grounds.

It targeted the chaotic evacuation from Kabul airport in 2021 with a suicide bomb, killing 170 Afghans and 13 US servicemen.

The following year it targeted the Russian embassy in Kabul, killing at least six people and injuring others.

The group has carried out indiscriminate attacks on a maternity ward, bus stations and policemen.

In January this year, IS-K carried out a double bombing of a shrine in Kerman, Iran, killing nearly 100 Iranians.

In Russia it has carried out numerous small-scale attacks, the most recent being in 2020 – and already this year the FSB, Russia’s internal security service, says it has stopped several terror plots.

According to Russian state media at least three of the men are Tajiks from the Central Asian republic of Tajikistan, which used to be part of the Soviet Union.

It is obvious from their battered and bruised appearance in court that they have been especially harshly interrogated to the point of torture.

The problem with that is according to international norms, their confessions will be worthless – people will say anything to make the pain stop, including confessing to a narrative that is simply untrue.

Another report says at least two of the attackers arrived in Russia recently, implying that this was a “hit team” sent by IS-K, rather than a sleeper cell of residents.

There are several reasons.

IS-K consider most of the world to be their enemies. Russia is high up on their list, along with the US, Europe, Israel, Jews, Christians, Shia Muslims, the Taliban and all rulers of Muslim-majority states, who they consider to be “apostates”.

Islamic State’s hostility to Russia goes back to the Chechen wars in the 1990s and early 2000s, when Moscow’s forces devastated the Chechen capital Grozny.

More recently, Russia entered the Syrian civil war on the side of its ally, President Bashar al-Assad, and the Russian air force has carried out countless bombings of rebel and civilian positions, killing large numbers of Islamic State group and Al-Qaeda-linked fighters.

In Afghanistan, IS-K view Russia as being an ally of the Taliban, which is why they attacked the Russian embassy in Kabul in 2022.

They also bear a grudge for the 10 years of brutal Soviet occupation of that country from 1979-89.

Then there is the situation inside Russia itself.

Russia is viewed by IS-K as very much a Christian country and their video posted after the Moscow attack talks about killing Christians.

Tajik and other Central Asian migrant workers are sometimes subject to a degree of harassment and suspicion by the FSB as it seeks to head off terrorist attacks.

Finally, Russia – a nation currently distracted by its full-scale war with its neighbour Ukraine – may simply have been a convenient target of opportunity for IS-K, a place where weapons were available and their enemy’s guard was down.

There remain a number of unanswered questions about this whole episode.

For example, why were the attackers able to wander at will around the Crocus Hall with absolutely no apparent sense of urgency?

In a country where the police and special services, notably the FSB, are omnipresent, these gunmen behaved as if they knew they were not going to be interrupted by a police SWAT team.

Then there are the weapons – not just handguns but powerful, modern automatic assault rifles. How were they able to acquire these and smuggle them undetected into the venue?

Unlike many jihadist gunmen on a raid like this, these men were not wearing suicide vests or belts, in the manner of those who prefer death to capture.

And yet, it did not take long for the Russian authorities – the same Russian authorities who failed to stop the worst terror plot in 20 years unfolding beneath their noses – to round up the suspects and put them on trial.

All this is prompting some analysts to speculate about some sort of so-called “inside job” by the Kremlin, or a “false flag operation” to garner popular support for the war on Ukraine.

However, there is no hard evidence to support that theory and US intelligence has confirmed that in their view, it was Islamic State behind this hideous attack.