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CIA: Saudi attacks bear hallmarks of Islamic State
Seven people were killed and two injured in the bomb blasts that shook Medina, a Shia mosque in Qatif and Jeddah last week, but not group has yet claimed responsibility for the attacks.
However, observers swiftly noted that the methods used resembled those of the Islamic State group, with Brennan suggesting "those three attacks were, I think, the work of ISIL."
"ISIL presents a very, very serious threat not just to Europe and the United States... but inside of Saudi Arabia," he told an audience at the Brookings Institution.
Nineteen suspects were arrested several days after the attacks, the Saudi interior ministry announced, 12 of which were Pakistani nationals.
The kingdom – which is part of the US-led international coalition fighting IS across Iraq and Syria – vowed to hit hard on radicals seeking to harm young people in response.
The Gulf State "is determined to strike with an iron fist all those who want to harm the spirit and orientation of our youth," King Salman said during a speech to mark Islam's Eid al-Fitr – a holiday to celebrate the end of the holy fasting month of Ramadan.
King Salman, 80, also cautioned young people against "the dangers that lie in wait, notably extremism."
Authorities have clamped down on radicals recently, dismantling an IS-linked group and detained hundreds of mainly Saudis in 2015.
Saudi Arabia's top cleric, Sheikh Abdulaziz al-Sheikh, has declared the IS group to be an "enemy of Islam."
More than half of the Saudi population are under 25 and several have declared allegiance to the Islamic State group.
Furthermore, documents leaked in May suggest 362 Saudi nationals travelled to join the group in combat.
A large number of them hailed from central Saudi Arabia, from areas like Riyadh and Hail; and the Western Province, where Mecca and Medina are located.
Most members – 313 – came from major cities like Dammam, Riyadh, Mecca, Jeddah and Buraydah.
One hundred fifteen said on their forms they were from Riyadh, and 75 from al-Qassim, including 60 from Buraydah alone.