ISIS’s claims of responsibility have in the past generally proved at least somewhat credible—though the extent of the group’s actual oversight of an attacker is rarely clear. Some high-profile attacks, such as the ones in Paris in 2015, were designed and executed by the group; others, such as the one in San Bernardino, California, that same year, involved militants inspired online by ISIS’s propaganda. More recently ISIS has issued more dubious claims, including for last year’s Las Vegas shooting. If indeed ISIS is found to be behind Saturday’s attack, it shows that the group, which has long wanted to target Iran, retains the ability to strike inside the Islamic Republic, a Shia country where ISIS, a Sunni group, ostensibly has little support. This isn’t the first time ISIS has struck in Iran. In June 2017, its militants targeted Iran’s Parliament and the mausoleum of Ayatollah Ruhollah Khomeini, killing at least a dozen people and wounding more than 40. What makes the most recent attack much more dangerous, besides the higher death toll, is that it feeds into a bigger regional conflict depending on how Iran responds: ISIS is active in both Syria, where it is being targeted by the U.S., and in Iraq, where it continues to hold influence despite being defeated in its major urban strongholds last year.

The wars in Iraq and Syria have brought together a dizzying array of alliances in which one country may be tacitly allied with another in Iraq but at odds with it in Syria. But almost all the countries involved in the region agree on two things: They oppose ISIS and they oppose Iran (which oppose each other). An ISIS attack on Iran complicates matters for all of those actors.

ISIS never went away in Iraq.

If it is shown that the little-known Ahvaz National Resistance is behind Saturday’s attack, as Tehran maintains, the implications could be no less severe. Iran says the Arab group is funded and trained by Saudi Arabia and the United Arab Emirates, the two countries that, along with Israel, have been at the forefront of confronting the Islamic Republic’s policies in the region. Both those countries deny supporting the group. The group seeks a separate state in the oil-rich Khuzestan province, where the city of Ahvaz is located. Its leaders, many of whom live in European countries, did not provide direct evidence to bolster its claim.

There were two predictable reactions in the aftermath of Saturday’s attack: Iran’s president, who will address the UN General Assembly this week, blamed the U.S. and its allies, and Haley, the U.S. ambassador to the United Nations, replied: “I think the Iranian people have had enough, and that’s where all of this is coming from. But, having said that, he can blame us all he wants, but the thing he’s got to do is look in the mirror.”