Saudi King Salman bin Abdulaziz vowed justice on Sunday against the perpetrators of the suicide bombing last week that left 21 dead, in an attack against a Shi’ite minority mosque during Friday prayers.

“We were pained by the enormity of the crime of this terrorist aggression which contradicts Islamic and humanitarian values,” Salman said in a message to Interior Minister Crown Prince Mohammed bin Nayef. “Any participant, planner, supporter or sympathizer with this heinous crime will be held accountable, tried and will receive the punishment he deserves.”

In the attack the bomber, identified by Saudi Arabia on Saturday as Saudi national Salih bin Abdulrahman Salih Al Ghishaami according to Al Arabiya, detonated himself during prayers at the Imam Ali mosque in the eastern town of al-Qadeeh, wounding 81 in addition to the 21 dead.

The attack, which took place in the Qatif governorate where the Sunni-majority country’s Shi’ite minority largely resides, was claimed by a local cell of the Sunni terror group Islamic State (ISIS), according to Saudi Interior Ministry spokesperson Maj. Gen. Mansour al-Turki.

The Interior Ministry revealed that the suicide bomber “was wanted by security services for belonging to a terrorist cell receiving directions from Daesh abroad,” using an Arabic acronym for ISIS.

It added that the ISIS-affiliated cell “was discovered last month, and so far 26 of its members, all Saudi nationals, have been arrested.” Likewise, Al-Qishaami’s father was “detained for a while,” according to al-Turki.

Saudi Arabia also announced that five members of the same ISIS cell were behind the murder of a Saudi soldier two weeks ago.