DUBAI — Saudi Arabia’s Interior Ministry says four security officers were killed and five others were wounded when a suicide bomber detonated his vest outside one of Islam’s holiest sites.

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 sprawling mosque where the Prophet Muhammad is buried is visited by millions of Muslims from around the world each year during pilgrimages to Mecca. The area was packed with pilgrims for prayer during the final days of the Muslim holy month of Ramadan, which ends in the kingdom on Tuesday.

Altayeb Osama, a 25-year old Sudanese visitor to Medina and resident of Abu Dhabi, said he heard two large booms about a minute apart as he was heading toward the mosque for sunset prayers. He said police and fire trucks were on the scene within seconds.

“It was very shocking that such a thing happens in such a holy place for Muslims, the second holiest place in the world. That’s not an act that represents Islam,” Osama said. “People never imagined that this could happen here.”

Qari Ziyaad Patel, 36, from Johannesburg, South Africa, was at the mosque when he heard a blast just as people were breaking their fast with dates. Many at first thought it was the sound of traditional, celebratory cannon fire, but then he felt the ground shake.

“The vibrations were very strong,” he said. “It sounded like a building imploded.”

Saudi Arabia’s state-run news channel al-Ekhbariya aired live video of thousands of worshippers praying inside the mosque hours after the explosion. It also showed footage of SaudiKing Salman’s son and the Governor of Medina, Prince Faisal bin Salman, visiting security officers wounded in the blast and the site of the explosion.

Also Monday evening, at least one suicide bomber and a car bomb exploded near a Shiite mosque in eastern Saudi Arabia, according to a resident there, several hours after another suicide bomber carried out an attack near the U.S. Consulate in the western city of Jiddah.

Saudi Arabia has been a target of Islamic State attacks that have killed dozens of people. In June, the Interior Ministry reported 26 terror attacks in the kingdom in the last two years.

The possibility of co-ordinated attacks across different cities in Saudi Arabia on the same day underscores the threat the kingdom faces from extremists who view the Western-alliedSaudi monarchy as heretics and enemies of Islam. Saudi Arabia is part of the U.S.-led coalition fighting the Islamic State group in Iraq and Syria.

The attack in the eastern region of Qatif did not appear to cause any injuries, said resident Mohammed al-Nimr. His brother, prominent Saudi Shiite cleric Nimr al-Nimr, was executed in January after a court found him guilty of sedition and inciting violence for his role in anti-government protests.

Qatif is heavily populated by Shiites, who are a minority in the Sunni-ruled kingdom. Al-Nimr said that near the body of a suicide bomber was a car bomb that also went off around the same time. He told The Associated Press the bomber detonated his suicide vest when most residents of the neighbourhood were at home breaking the daily Ramadan fast.