BEIRUT (Reuters) - An explosion at a guesthouse near the Fatima Masumeh shrine in the Iranian city of Qom on Thursday left four dead and 15 wounded, most of them Iraqi pilgrims, according to the semi-official Mehr news agency. The explosion was caused by a pilgrim mishandling a leaking cooking gas cannister, Mehr said, citing local witnesses. A spokesman for the Qom fire department told Mehr the cause is still being investigated. Some of the dead and wounded may still be under rubble inside the guesthouse, according to Mehr. Photos posted by Mehr showed bloodied pilgrims laid out in the street and firemen rummaging through the ruins of the guesthouse. The shrine in Qom, which is approximately 150 kilometers (90 miles) south of the capital, Tehran, is the second-most sacred spot in Iran for Shi ite Muslims. 