(CNN) -- Inadequate security measures and an unprepared military are two factors that allowed a group of Islamists to attack and take hostages at a gas field in Algeria in January, a report says.
The report by Statoil, whose employees were among those taken hostage, says the companies that ran the gas field never anticipated a scenario where a large force of attackers hit the facility.
The companies were too dependent on the Algerian military, which was "not able to detect or prevent the attackers from reaching the site," the report says.
At least 37 hostages were killed in the four-day ordeal.
The plant is run by In Amenas Gas, a joint venture between oil giant BP, Statoil, and Sonatrach.
Among the victims were five Statoil employees, which led the company to conduct an investigation into the circumstances surrounding the attack.
"The terror attack against In Amenas was an unprecedented attack," lead investigator Torgeir Hagen said in a statement. "It clearly demonstrates that also companies like Statoil today face serious security threats."
The investigation team concluded that a different response to the attack would not have changed the outcome.
But the lesson is that the site was not designed with security measures that would stop or slow an attack on that scale, the report says.
The militant siege caught the world's attention as it ensnared citizens from several nations and dragged on for days.
Algerian authorities said they believe the attack was revenge for allowing France to use Algerian airspace for an offensive against Islamist militants in neighboring Mali.
A group led by the jihadist commander known as "Mr. Marlboro," Moktar Belmoktar, was responsible for the deadly attack.
The plant in southern Algeria employed about 790 people, including 134 foreign workers.