At least 28 pilgrims from Pakistan were killed in Iran when their bus overturned on Tuesday night while traveling to commemorate a Shiite Muslim holiday, state-run media in Iran has reported.

The bus was heading to the Iraqi city of Karbala for Arbaeen, a two-day holiday that begins on Sunday evening, draws tens of millions each year and is one of the world’s largest organized gatherings.

At least 23 other passengers were wounded, 14 of whom were critically injured, the state-run news agency IRNA reported, adding that Iran was working to repatriate the bodies and fly the injured back to Pakistan.

Many of the pilgrims came from Sindh Province in the south of Pakistan, Pakistani officials and local news media said.

Video broadcast on state-run Iranian television showed a yellow and red bus lying upside down. Its big wheels were up in the air. The doors to the luggage storage compartments under the bus had fallen open. Men looked around a gnarl of metal and machinery with flashlights, searching.

The accident took place in Yazd, a province in central Iran, the state media said. The journey from Pakistan, through Iran and on to Karbala can extend 1,100 miles. The cause of the crash was not immediately clear.