Within hours of the mass shooting at a crowded Moscow concert venue that left at least 133 people dead and the hall in flames on Friday, the Islamic State terror group claimed responsibility for the attack.

United States intelligence soon confirmed the involvement of the group, commonly known as ISIS.

The massacre has specifically been linked to the group's ISIS-K branch.

Here is information about ISIS, ISIS-K and the Moscow attack:

WHAT IS ISIS?

The Islamic State - also known as the Islamic State of Iraq and Syria (ISIS) or Islamic State of Iraq and the Levant (ISIL) - is a jihadist terror group that gained international infamy in the mid-2010s, when its militants seized large swaths of Iraq and Syria.

Around this time, the group was particularly notorious for its graphic video recordings of beheadings and terror attacks around the globe.

But by 2019, American and other military forces had largely reclaimed the territory captured by ISIS, significantly reducing its power and returning it to launching insurgent strikes from the shadows.

WHAT IS ISIS-K?

ISIS-K, or Islamic State - Khorasan Province, is a local branch of ISIS primarily based in Afghanistan since its establishment in late 2014, according to Reuters.

"Khorasan" refers to a historic name for the region encompassing parts of Afghanistan, Iran and Turkmenistan.

Through the years, it has launched a number of high-profile attacks - primarily bombings - against civilians and, more recently, battled the Taliban government in Afghanistan.

WHY WOULD ISIS-K ATTACK RUSSIA?

ISIS-K has increasingly directed its ire in recent years toward Russia and newly-reelected President Vladimir Putin.

The group "sees Russia as being complicit in activities that regularly oppress Muslims," Michael Kugelman of the Washington-based Wilson Center told Reuters.

Additionally, the group's members include Central Asian militants who may bring their own regional grudges against Russia to the table.

"ISIS-K has been fixated on Russia for the past two years, frequently criticizing Putin in its propaganda," Colin Clarke of the Soufan Center, another Washington-based research group, told Reuters.

Putin reportedly received recent warnings from the United States that Moscow could be targeted by a terror attack, but dismissed the heads-up as "blackmail" with "the intention to intimidate and destabilize our society."