US officials claim that this past weekend's attack on a crucial Saudi oil facility came from Iran.

If so, this represents a massive failure of the US to notice or intercept the attack.

The US and Saudi Arabia have spent billions preparing for a scenario just like this, with ships, radar arrays, and air defenses ready to go.

But none of this seems to have worked. At least 17 strikes are believed to have connected, knocking out 5% of the world's oil-production capacity at a stroke.

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It has yet to be definitively established how the massively disruptive attacks this past weekend on a crucial Saudi oil facility took place.

The version of events being advanced by US officials, however — that most of the damage was from cruise missiles launched from Iran — raises the embarrassing question of why the US military was unable to do anything about it.

The airspace around Iran and Saudi Arabia is some of the best-defended and most intensively monitored on earth, thanks to the decades-long buildup of US assets there. But on Saturday those defenses failed to prevent what US officials have said were at least 17 separate strikes.

Based on information made public about the strikes, defense insiders were left wondering how the US military had fared so poorly in one of its primary missions in the region.

Storage tanks at the North Jiddah bulk plant, an Aramco oil facility, in Jiddah, Saudi Arabia, on Sunday. Associated Press

One former US Navy officer, who deployed to the Persian Gulf region twice to operate air-defense systems, said it would be nearly impossible for the US not to notice the attack as it happened or attempt to intercept the weapons.

"It's very hard to imagine a salvo of 17 shots from Iranian territory not being picked up via some land and sea radars," said the former officer who asked not to be identified discussing US military capabilities in the region.

"Over the Persian Gulf is hard to comprehend ... in that there'd be a lot of radars to detect it. There may be ships in-port [in] Bahrain whose air-defense radar would pick it up."

The attack Saturday struck two key oil facilities in the energy-rich eastern part of Saudi Arabia and knocked out about 5% of the world's oil production.

An annotated image provided Sunday by the US government and DigitalGlobe showing damage to the infrastructure at Saudi Aramco's Abaqaiq oil-processing facility in Buqyaq, Saudi Arabia. AP

The attack was immediately claimed by Houthi fighters in Yemen, who are closely linked to Iran and who have regularly targeted Saudi facilities with drones and cruise missiles during the nearly five-year-old Saudi involvement in Yemen's civil war.

But while the targets appear to be within range of the Houthi-controlled parts of Yemen, some US officials told reporters Monday that the attacks most likely originated from Iran. The US government has yet to provide conclusive evidence.

Experts, however, say that such a strike launched out of Iran should create plenty of hard evidence, via records from regional radar and air-defense systems. The US should also have been able to do something about it.

Smoke seen following a fire at an Aramco factory in Abqaiq on Saturday. VIDEOS OBTAINED BY REUTERS/via REUTERS

Multiple experts speaking on background characterized the US presence in the area as having three main goals: defending US military bases, keeping the Persian Gulf open to shipping, and defending Saudi oil facilities from attack.

Assets to monitor attacks from Iran are considerable: a band of radars based in Kuwait, Qatar, and Bahrain are all pointed at Iran to provide early warning. Many US ships are in the area, equipped with the Aegis air-defense system, which has a range of some 300 nautical miles.

US land bases are also protected by similar, albeit shorter-range, systems. Saudi Arabia itself has spent billions on its air force and missile defense, including top-of-the-line US-made Patriot air-defense systems specifically meant to defend against Iran.

People next to a Royal Saudi Air Force Eurofighter Typhoon jet during the opening ceremony of the Bahrain Air Show in Sakir Base, south of Manama, in 2012. REUTERS/Hamad I Mohammed

"Oversea shot seems unlikely — Bahrain has a long-range air-surveillance radar," the former officer said, including the sophisticated AN/TPS-59 radar. "Seems unlikely that something like that would miss some ballistic missiles."

There has been no evidence that US or Saudi radar systems picked up the incoming attack or that either military attempted to intercept the missiles before they struck the facilities.

But US officials Monday quietly indicated to reporters in Washington that they believed the attacks came from Iran. They said they were deferring to the Saudis on making such an announcement, because such a strike could lead to a war.

The only evidence thus far produced by the US has been satellite imagery that showed the impacts mostly came from the west — away from Yemen to the south and Iran to the north and east.

"Those photos only prove that the cruise missiles used that approach on target — they tell us nothing at all about where they originated from," said a NATO military official recently posted to Saudi Arabia.

"If these came from Iran, the Americans can easily prove it with radar data that they haven't yet released.

"If they haven't released that info, it's because either they don't have it or the Saudis asked for a delay for domestic political reasons."