Dozens have been killed and more have been injured in a bombing at a crowded mosque on Thursday, and officials expect the death and injury tolls to rise.

The Balili mosque, located in Yemen’s capital of Sanaa, was hosting several worshippers who were saying prayers to mark the Muslim holy day of Eid al-Adha at the time of the two explosions. Officials reported that the suicide bomber housed a bomb in his shoe, which he detonated, and then ran into the middle of the crowd to detonate the second bomb. Security also found two other devices in the bathroom that had not detonated.

“We found a shoe bomb and explosives hidden in underwear and abandoned in the toilet,” said Adnane Khaled, the head of security at the mosque.

The Islamic State claimed responsibility for the attack on Twitter. The mosque is located in an area controlled by Shiite rebels, which ISIS, a Sunni militant group, views as heretics. This attack is the sixth time in three months that the mosque has been a target.

The attack comes two days after Yemen’s President Abdrabbuh Mansour Hadi returned from exile. He had fled the country in March when Houthi rebels made gains.

The United Nations estimates that thousands of civilians have been killed in Yemen since March. The number varies between 2,200 and 5,000.