Gunmen opened fire on a memorial service in a Shiite Muslim graveyard in Lahore on Sunday, killing as many as 22 people and injuring more than 30 others, police said.

Several of the wounded were in critical condition, officials said.

The shooting in the heart of Pakistan’s eastern Punjab capital may have been religiously motivated. Police believe the gunmen belong to a militant Sunni group known as Friends of the Guardians of the Prophet. However no one has taken responsibility for the killing.

Nearly 100 Shiite Muslim worshipers had gathered at the graveyard to honor dead relatives when two gunmen on a motorcycle drove up and opened fire with automatic weapons, police said.

People scrambled for cover, but before the shooting stopped several minutes later, 22 people were dead and another 30 wounded.