Baghdad — The Islamic State said Saturday it used a truck bomb packed with three tons of explosives to target Shiite pilgrims as they walked to a shrine in Baghdad, in an attack that killed at least 23 people.

The bomber, however, was unable to reach the pilgrimage route as roads in the area were closed to traffic, said Haider Abdullah, mayor of the Baghdad neighborhood of Nahrawan, where the attack took place.

Instead, the suicide bomber detonated the truck bomb among crowds at a busy sheep market nearby, Abdullah said.

Thousands of Shiite pilgrims from across Iraq are making their way on foot to the shrine of eighth-century Imam Moussa al-Kadhim to commemorate his death. Such gatherings are a target for the Islamic State, which considers Shiite Muslims as apostates and regularly bombs their neighborhoods.

Extra security measures including roadblocks and checkpoints had been put up in recent days in the capital in an attempt to secure the pilgrimage route.

In a statement circulated on social media, the Islamic State said it targeted pilgrims on the way to their “polytheist ritual.”

“For sure it was supposed to target the pilgrims,” said Abdullah, the mayor. “But when they found all the roads were closed, they chose this place because it was very crowded.”

He said 43 people were killed in the bombing about 6:30 a.m.; Iraq’s Interior Ministry put the death toll at 23. The Islamic State claimed it had killed “nearly 100.”

Abdullah, who had visited the bomb site, said it had left a deep crater, bursting the water pipe in the ground below. Electricity and water supply in the area had been disrupted, he said.