Iraqi officials reported that at least nine people were killed and an additional 21 were wounded when Shiite Muslims taking part in an annual pilgrimage in Baghdad were targeted by a suicide bomber, according to the Washington Post.

The attack occurred Monday at a checkpoint along a route used by Shiite pilgrims in the northern Baghdad neighborhood of al-Shaab, marking the first attack on Shiite pilgrims in Baghdad during the beginning of Arbaeen, when pilgrims go to Karbala, according to ABC News.

Since Arbaeen is a Muslim religious observance that has influenced various political uprisings in the past, it's frequently marked by violence despite security measures taken to protect the pilgrims.

Officials revealed that four soldiers stationed at the checkpoint were among the casualties.

No one has claimed responsibility for the attack, reported the International Business Times. However, suicide bombings like this are almost exclusively done by Sunni extremists throughout Iraq, most notably ISIS, who view Shiites as heretics who have strayed from the "true" Islam.

The attack comes a day after security forces in Baghdad foiled an attempted suicide bombing by killing the would-be attacker who was targeted Shia pilgrims visiting the south of Baghdad.