It is precise journalistic language. The writer is a reporter. Reporters are supposed to report facts, not express their own opinions. Was it a terrorist attack? The reporter doesn't know. They know what certain people _said_. So they don't say people were killed "in a terrorist attack." Notice the levels of word grouping in this sentence. "State media" is a unit. It means the official government media of the country. "State media and regional authorities" is a large unit. This is the set of people who called it a terrorist attack. The whole phrase "what state media and regional authorities described as a terrorist attack" is a careful phrase that makes it clear that they are just reporting what someone said. They are not saying it is true. It is followed by another long phrase. The reporter mentions the explosions, and then follows it with two long phrases: "in what X described as Y, and which Z blamed on W." There are two sides to the story. State media and regional authorities said one thing. The government in Tehran said something different. The journalist is try to write one long sentence that indicates, very quickly, that there are two sides to the story. The journalist is not taking sides, but is reporting what authorities said.

it refers to Iran... It means the state media and regional authorities of/in Iran described it as a terrorist attack. It is not a question asking in what state?. if it was then there would be a ? and it would probably be worded a little differently. Around 100 people were killed by two bomb explosions in Iran, state media and regional authorities described it as a terrorist attack and which the government in Tehran later blamed on America and Israel. Another way to write it is remove "in what" and add "it" after described. like in the above example. Or to ask what state/ nation reported as a terrorist attack. Around 100 people were killed by two bomb explosions in Iran, in what state? did media and regional authorities describe it as a terrorist attack. We are still waiting for confirmation. The state and regional authorities of Iran are reporting two bomb explosions in Iran and describing them as terrorist attacks. The state and regional authorities of Iran are reporting two bomb explosions in Iran in what they are describing as terrorist attacks.