One of two attackers who wore explosive belts in Friday’s deadly assault on a Shiite mosque in the country’s east was a 22-year-old Saudi national, Saudi Arabia’s Interior Ministry announced.

Saudi Interior Ministry spokesman Maj. Gen. Mansour al-Turki, in a statement released Saturday night, identified the suicide bomber as Abdulrahman bin Abdullah bin Suleiman al-Tuwaijri. He had been briefly detained in 2013 in the Saudi city of Buraydah during a protest to demand the release of imprisoned Saudis accused of fighting abroad and having ties to extremist groups.

Al-Tuwaijri died when he detonated his suicide vest at the entrance of the Imam Reda Mosque in al-Ahsa during Friday prayers. A second attacker was detained after an exchange of gunfire with police. He was not identified.

Al-Turki said four Saudis were killed and 33 people were wounded in the attack. Fourteen of those wounded are still receiving treatment for their injuries, he said.

No group has claimed responsibility for the attack, but Islamic State militants have targeted the kingdom’s minority Shiites in the past.

Saudi Arabia’s Interior Minister and Crown Prince Mohammed bin Nayef visited the National Guard hospital in al-Asha on Saturday, where some of the wounded are being treated. He was quoted saying that what happened, “will only strengthen our resolve in this country to eradicate this aggressor.”

Also, a state-linked Saudi newspaper reported that at least four U.S. citizens are among 41 terror suspects detained in recent days in the kingdom.

The Arabic-language al-Watan newspaper, quoting unnamed security officials, reported on Saturday that the four U.S. citizens were detained on Tuesday, bringing the total number of U.S. citizens detained in Saudi Arabia since 2007 up to nine. The newspaper said one of the nine U.S. citizens was convicted by a Saudi court in 2007, while the rest are still being investigated.

No further details were given about whether the U.S. citizens in custody are dual Saudi nationals.