31 were killed in an explosion at a Shi’ite Muslim mosque in Pakistan’s capital, Islamabad, on Friday, police said.

The blast occurred during Friday prayers, according to police official Zafar Iqbal. “We have shifted several people to hospitals. I can’t say how many are dead at this moment, but yes, people have died,” he said.

Authorities have not yet determined the type or cause of the explosion. Police are present at the site as investigations continue.