Iran’s Revolutionary Guards say Saudi Arabia was behind the twin attacks in Tehran.
The deputy head of the powerful IRGC also said Iran will seek revenge for the deadly bomb and gun attacks.
A statement published by the Guards said the attack happened only a week after the meeting between the U.S. president Donald Trump and Saudi leaders.
It said the fact that Islamic State has claimed responsibility proves that they were involved in the attack.
The attacks were the first claimed by the I.S. in the tightly controlled Shi’ite Muslim Iran.
The Islamic State said in a statement on Wednesday that five of its fighters were responsible for the raid on Iran’s parliament and the shrine of Ayatollah Khomeini, using assault rifles, grenades, and suicide vests.
The Sunni militant group also threatened Iran’s majority Shi’ite population with more attacks, saying the caliphate would continue attacks until Sharia law is implemented.