Saudi Arabia identified the suicide bomber behind a deadly attack on Shiite worshippers in the oil-rich Eastern Province as a 20-year-old linked to an Islamic State terrorist cell.

Saleh al-Qashami was wanted for belonging to an Islamic State affiliate in the kingdom, the Riyadh-based Saudi Press agency said, citing an unidentified interior ministry spokesman. The explosive used Friday at Qudeih in Qatif was a military- grade compound known as RDX, SPA said.

The attack comes as Saudi Arabia participates in U.S.-led operations against Islamic State in Syria and leads a military intervention against Shiite rebels in Yemen. Islamic State claimed responsibility for the suicide bombing that killed 21.

“We have to find out who else is behind this,” Interior Ministry spokesman General Mansour al-Turki said in a phone interview. “We are sure he didn’t act alone.”

Using al-Qashami, whose father was jailed for terrorist- related activity, in a suicide attack represents an escalation in tactics by militants inside the Arab world’s biggest economy. They have already stepped up the frequency of strikes against police in the capital Riyadh.

Friday’s explosion was the second of its type on the Saudi Shiite community since November, when three assailants used automatic weapons and handguns to kill five people and wound nine. Islamic State warned of more strikes against the kingdom’s Shiite minority, who mostly live near some of the world’s largest oil fields in the eastern region.

Shiites Targeted

Saudi Shiites have complained about discrimination by the Sunni rulers, occasionally staging protests. Islamic State, the Sunni group that controls parts of Syria and Iraq, has repeatedly targeted Shiites, whom it considers heretics.

A five-member terrorist cell killed the commander of a security patrol in south Riyadh this month, the news service said. Police seized rifles, ammunition and chemicals for explosives when they raided a farm used by the cell, it said.

Saudi Arabia has vowed to crack down on Sunni extremist groups even as it’s accused by Shiite regional rivals of inciting them. Jihadists such as Islamic State and Al-Qaeda typically denounce the Saudi royal family for betraying Islam through allying with the U.S.

Islamists who returned to Saudi Arabia from wars in Iraq and Afghanistan have skills they later turned against the Al Saud family. Militants struck an oil installation and stormed a housing complex in al-Khobar in 2004, killing 22 foreign workers.

Saudi Arabia said last month it detained two people plotting to attack the U.S. embassy in Riyadh. Security forces have arrested 93 on terrorism-related charges in recent months, Interior Ministry spokesman Mansour al-Turki said in April.

In response, the U.S. embassy has restricted personnel from traveling to some Saudi locations, according to a security message for citizens. The areas include Awwamiya and al-Ahsa in the Eastern Province, the embassy said Saturday.