The Islamic State group on Saturday claimed a bomb blast that killed 12 people near the Iraqi Shia holy city of Karbala the previous day.
The blast aboard a bus at a checkpoint north of Karbala also wounded five people, according to the city’s health authorities.
Security forces said Saturday they had arrested a man suspected of placing the explosives on the bus before it disembarked.
Iraq declared victory against IS in late 2017 after three years of a brutal fightback against the extremist Sunni group, which had specifically targeted Shia gatherings.
Jihadist sleeper cells have continued to carry out hit-and-run attacks against government positions across the country, particularly at checkpoints, but attacks targeting Shia religious gatherings had been rare in recent years.
The deadliest incident this year was a stampede earlier this month in Karbala that left more than 30 pilgrims dead and dozens injured.
Hundreds of thousands of pilgrims from Iraq, Iran and as far as India had been gathering in the southern city this month to mark the Shia holy day of Ashura.
Iraq is expecting millions more Shia pilgrims to arrive at the end of October for the annual Arbaeen commemoration, which marks the end of the 40-day mourning period for the seventh-century killing of Imam Hussein by the forces of the Caliph Yazid.