Iranians gather near the scene of a bomb explosion in a mosque which killed several people, in the city of Shiraz

Iranian officials say investigators have not yet found the cause of a mosque explosion that killed at least 12 people and wounded 200 others.

Some officials are calling Saturday's blast in southern Iran an accident, while others are refusing to rule out a bomb attack.

The explosion happened in the men's section of a crowded mosque in Shiraz while a prominent cleric was delivering a sermon.

Provincial police chief General Ali Moayyedi said Sunday that the blast may have been caused by negligence.  He said explosives were left at the mosque from a recent exhibition commemorating the Iran-Iraq War of the 1980s.

Iranian media quote other officials as saying investigators are not ruling out the possibility of a terrorist attack.

The mosque's Shi'ite cleric had been preaching against a radical form of Sunni Islam called Wahhabism, which is practiced mostly in Saudi Arabia. The cleric also spoke out against the Bahai faith, which is banned in Iran.

Deadly bomb attacks in Iran have been rare in recent years.  The last major attack was in February 2007, when Sunni militants set off a bomb that killed 11 Iranian Revolutionary Guards in the southeastern city of Zahedan.

Some information for this report provided by AP, AFP and Reuters.