ISIS has claimed responsibility for the attack.

Shia worshippers had gathered to observe Arbaeen, a religious festivity which commemorates the 40th day of the martyrdom of Imam Hussein, whom they consider the rightful heir to the prophet Muhammad. The attacker arrived on foot and blew himself up just as the prayer ceremony was ending.

This is the third and most deadly attack on Shia congregations in the last four months: in October, a gun attack on another Shia mosque in Kabul killed at least 14 people; back in July, at least 80 people lost their lives in a suicide bomb attack which had targeted a Shia protest. ISIS also claimed responsibility for these attacks.

Although Afghanistan has so far not experienced sectarian violence on the same level  that Pakistan and Iraq have, many consider the latest events as attempts to foment discord, thus threaten the already fragile national unity. It is estimated that about 20% of the Afghan population are Shia Muslims, while the rest are Sunni.

To know more, read:

https://www.theguardian.com/world/2016/nov/21/kabul-mosque-hit-by-deadly-suicide-bomb-attack
http://www.bbc.com/news/world-asia-38048604
http://unama.unmissions.org/unama-condemns-killing-civilians-kabul-mosque-attack-0
http://www.un.org/apps/news/story.asp?NewsID=55614#