A series of suspected drone attacks Saturday that targeted oil facilities in Saudi Arabia resulted in explosions and fireballs, knocking out half the kingdom’s oil output for days. Now, questions are being asked about the extent of the damage and how the attack was carried out.

The key question, however, is who was responsible. Yemen’s Houthi rebels, at the center of a civil war against Saudi-backed forces, have claimed responsibility; on Monday, they threatened additional attacks.

But Western and Saudi officials have cast doubt on the claim, saying the attack did not originate in Yemen. They have instead pointed at a known backer of the Houthis: Iran.

“Amid all the calls for de-escalation, Iran has now launched an unprecedented attack on the world’s energy supply,” Secretary of State Mike Pompeo said in a tweet Saturday. “There is no evidence the attacks came from Yemen.”

The relationship between Iran and the Houthis is not simple and has long been clouded by accusations and denials and amplified by rumors and propaganda from all sides.

Who are the Houthis?

Based out of Yemen’s northwest, the Houthis first came to international prominence in 2015, when they helped topple the government of Yemen’s president and regional U.S. ally, Abed Rabbo Mansour Hadi.

Their history, however, stretches back to the early 1990s, when a group called Shabab al-Muminin (the Believing Youth) worked to raise awareness about the Zaydi branch of Shiite Islam, which had dominated Yemen for centuries but was sidelined after a civil war in the 1960s.

Hussein al-Houthi, one of the leaders of the Believing Youth, began staging anti-American protests after the U.S.-led invasion of Iraq in 2003. When Houthi was killed by government forces in 2003, his supporters renamed their group after him and continued a shift from religious protest to armed insurgency.

From 2015 onward, the Houthis have participated in the civil war that has engulfed Yemen, primarily facing off against supporters of Hadi, who is also backed by a Saudi-led international coalition.

What are their links to Iran?

Iranian backing of the Houthis appears to have increased over time. But experts on Iran’s network of proxies say the Houthis are among the least dependent on Tehran for financial and military support and decision-making.

Although the Houthis began as a primarily local movement, and the theology of the Zaydi branch of Shiite Islam is significantly different from that practiced by the Islamic Republic of Iran, the group is part of a wide network of Tehran-supported armed factions in the Middle East.

A 2009 diplomatic cable sent by the U.S. Embassy in Yemen said that contrary to the Yemeni government’s claims that the group was being armed by Iran, “most analysts report that the Houthis obtain their weapons from the Yemeni black market” and from Yemen’s military.

In 2017, Reuters interviewed an unnamed Iranian official who said that the Islamic Revolutionary Guard Corps had held a meeting on ways to “empower” the Houthis. “At this meeting, they agreed to increase the amount of help, through training, arms and financial support,” the official said.

Iran has issued official denials of accusations that it is arming the Houthis, but intercepted weapons shipments in the Arabian Sea have yielded rifles, rocket launchers, antitank guided missiles and munitions that appear to have been en route from Iran to Yemen for the insurgency.

Have the Houthis targeted Saudi Arabia before?

Yes. Since the start of the conflict within Yemen, the Houthis have sought to punish Saudi Arabia for its prominent role by launching attacks on Saudi soil. Last year, Saudi officials said they had intercepted more than 100 ballistic missiles fired from Houthi territory.

Armed drones attacked oil-pumping stations west of Riyadh in May and caused serious damage, while an attack on Abha airport in the south wounded 26 people in June.

But Saturday’s attacks struck right at the center of Saudi Arabia’s oil production facilities, making it a significantly more sophisticated operation than what the Houthis have been known for in the past. The blasts took place in the districts of Khurais and Abqaiq, more than than 500 miles from the Houthi-controlled zones in Yemen, using precision strikes to cause maximum damage.

The attacks may have used both drones and missiles. Fabian Hinz, a research associate at the James Martin Center for Nonproliferation Studies at Monterey, argued that photos of the remnants of a missile in Saudi Arabia show a weapon both too sophisticated to be produced domestically by the Houthis and never seen in Iran.

“Is Iran secretly designing, testing and producing missile systems for exclusive use by its proxies?” Hinz asked in a blog post for Arms Control Wonk.

What if the Houthis didn’t do it?

The advanced nature of the attack has led to assertions that it did not originate in Yemen but was carried out by Iranian proxies in Iraq or even Iran itself.

It is not clear why the Houthis would claim the strike, if so. It may be part of a regional strategy by Iran and its allies that would attempt to sow confusion, although many analysts have argued in the past that the Houthis, driven by local concerns, act independently of Iran when they wish to.

“Now they say that they had nothing to do with the attack on Saudi Arabia,” President Trump tweeted of Iran on Monday morning, before adding a tentative question. “We’ll see?” he wrote.