Here’s what’s known — and what remains unclear — about Wednesday’s deadly explosions in Iran:

• Authorities say the twin bombings killed at least 103 people and wounded 211 others.

• The attack is the deadliest to strike Iran since its 1979 Islamic Revolution.

• The second explosion occurred about 20 minutes after the first, which is often a technique used by militants to harm both civilians and emergency responders after an initial assault.

• The attack came during commemorations for the fourth anniversary of the killing of Gen. Qassem Soleimani, the head of the Revolutionary Guard’s elite Quds Force, in a U.S. drone strike in neighboring Iraq. The explosions occurred near his grave site as long lines of people gathered for the event.

• No group has claimed responsibility for the attack in Kerman.

• Iran has faced targeted killings and sabotage attacks suspected to have been carried out by Israel. However, those assaults did not include mass casualty bombings like Wednesday’s attack.

• Sunni extremist groups including the Islamic State group have conducted large-scale attacks in the past that killed civilians in Shiite-majority Iran.

This is terrorism written all over it.