Inadequate quality controls on work performed on an upgrade project and a power grid unable to handle overloads were the main factors behind the massive Sept. 12 power outage that swept through much of Los Angeles, according to an independent report released Tuesday.

The midday power outage affected at least 700,000 Los Angeles Department of Water & Power customers in Los Angeles, as well as residents of Burbank and Glendale, whose utilities are linked to the L.A. system. All in all, some 2 million people lost power. The outage initially sparked fears of a terrorist attack, but as power was restored indications pointed towards human error, specifically a faulty work order at a substation in Toluca Lake.

Following the outage, the DWP commissioned Electric Power Group LLC to conduct an independent review of the events leading up to the outage and how the power outage spread from a single substation to much of the L.A. metropolitan area.

The 50-page report paints a tangled picture of problems that led to and exacerbated the outage. The “root cause” of the power outage was a combination of inadequate review of work orders on the DWP’s substation automation project and lack of quality control. This in turn led a DWP worker to pull the wrong set of cables, thereby triggering the outage.

Once the outage began, the report cited a series of DWP operator errors and system malfunctions that caused it to spread, including the shut-down of the Haynes Generating Station in Long Beach and the Scattergood Generating Station in Playa del Rey and the failure of peaking units to come on line to replace the lost power. The report also said that DWP grid operators were so overwhelmed by more than 1,500 alarms in the first three minutes that they were unable to determine which ones were the most urgent.

The report contained dozens of recommendations to prevent a repeat of the outage, including better controls on work procedures, redesign of the transmission grids into and out of major generating stations and filters to prioritize alarms.

Responding to the report, DWP General Manager Ronald Deaton said in a statement: “LADWP management and staff are seriously evaluating each and every recommendation in this report and we will take corrective actions as expediently as possible. Our primary concern is to do everything possible to ensure that an incident such as the Sept. 12 outage is not repeated.”