At least four security officers were killed and five injured in a suicide bombing Monday outside an Islamic holy site in Medina, according to the Saudi Interior Ministry.

The Monday evening attack took place just as thousands of worshippers were about to hold sunset prayers in the mosque where the Prophet Muhammad is buried in Medina.

The mosque is visited by millions of Muslims from around the world annually performing pilgrimage in Mecca.

The ministry statement, carried Monday on the state-run al-Ekhbariya news channel, said the attacker set off the bomb after security officers raised suspicions about him. The attack took place in a parking lot outside the sprawling mosque complex.

The bombing happened the same day that two suicide bombers struck different cities in Saudi Arabia, one of them near a U.S. consulate. No group has yet claimed responsibility for any of the attacks.

Qari Ziyaad Patel, 36, from Johannesburg, South Africa, was at the mosque when he heard a blast just as the call to sunset prayers was ending. People were breaking their fast with dates so the mood was subdued, he said. Many at first thought it was the sound of traditional, celebratory cannon fire, but then he felt the ground shake.

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