The report by Norway's Statoil, which operates the sprawling complex in the Sahara Desert with BP and Algeria's state-run Sonatrach, underlines the vulnerabilities of energy facilities in North Africa at a time of widening jihadist operations across oil-rich North Africa.
The dawn attack on In Amenas Jan. 16 marked the first time jihadists penetrated a major energy facility and held it.
The raiders had apparently planned to blow up key installations, but they were apparently overwhelmed before they could so by Algerian special forces after a four-day siege.
"The terror attack against In Armenas was an unprecedented attack," Statoil's investigation leader, Torgeir Hagen, observed in a statement. "It clearly demonstrates that companies like Statoil today face serious security threats."
The only other attack on such a target was the Feb. 24, 2006, assault by al-Qaida against the massive Abqaiq oil processing facility in Saudi Arabia with several massive truck bombs.
It failed, but had the attack on the world's largest oil-processing plant succeeded, it could have cut off more than half the kingdom's daily exports and caused turmoil in global markets.
The Algerians, North Africa's regional military heavyweight, were clearly caught flatfooted even though they've been fighting Islamist insurgents for more than two decades.
The Algerians have not yet provided a detailed account of events themselves, or explained how they failed to detect the attackers as they moved across the desert to hit In Amenas.
The Statoil probe said the assault was carried out by "a diverse group" largely made up of desert fighters from the Signed in Blood Brigade, which is headed by veteran Algerian jihadist Mokhtar Belmokhtar; an Algerian rebel group known as Sons of the Sahara for Islamic Justice; and facilitators from Libya.
Belmokhtar's group is an offshoot of al-Qaida in the Islamic Maghreb, the main jihadist force in the region. It was heavily involved in the fighting in nearby Mali against French troops and African forces.
Indeed, the attack on In Amenas was believed to be retaliation for the French intervention, Operation Serval, launched Jan. 11 that pushed the jihadists out of the sanctuary in northern Mali they had seized in 2012.
One of the leaders of the In Amenas attack was a Canadian national, who trained under Belmokhtar, Statoil said.
The report said most of the 32 attackers were Egyptians and Tunisians, underlining the regional make-up of the jihadist forces now operating from Morocco in the west, across the Sahara and the semi-arid Sahel belt all the way to Syria and Iraq.
The growing scale of this jihadist movement, and the swelling intensity of fighting in Egypt's Sinai Peninsula, Syria and Iraq, with an emerging presence in Lebanon, emphasizes how the Islamist threat is expanding.
The In Amenas attackers struck from their base in southern Libya, which has become a sanctuary for jihadists and other Islamist militants amid the anarchy that followed the 2011 civil war that toppled longtime dictator Moammar Gadhafi and flooded the region with weapons plundered from his armories.
Undetected, the attackers seized the poorly protected gas complex, crashing trucks through the plant's gates, hitting the production area and the living quarters simultaneously.
According to the Statoil report, it took the masked attackers only 15 minutes to seize control of the facility and the 800 people, 134 of them foreigners, working there.
It was evident the attackers had inside knowledge of the complex, its operational tempo and its poor security. They rounded up the foreigners, mainly Western technicians, who were handcuffed and strapped to explosives, the report said.
Later, when the Algerians unleashed their ferocious, take-no-prisoners assault on the complex, the jihadists used the foreigners as human shields as helicopter gunships strafed the facility.
When the smoke cleared Jan. 19, 40 foreign workers from 10 countries were dead along with 29 terrorists.
It remains to be seen whether the energy industry has absorbed the lessons of the In Amenas slaughter, including an over-reliance on Algeria's military, which Statoil stressed "was not able to detect or prevent the attackers from reaching the site."