The final report on the cause of a rolling blackout that all but shut down southern Ontario and eight U.S. states in mid-August won't be ready before Christmas.

Natural Resources Minister Herb Dhaliwal says the causes of the extensive power outage are multiple: an outdated grid system and inadequate electrical regulation appear to be likely factors.

The electrical shutdown was triggered by voltage fluctuations in Ohio, but ended up blackening the streets in cities in a wide swath that cut across New York state, Ontario and Michigan.

Herb Dhaliwal

An interim report is expected to be ready in three weeks.