The fire last October shut down the city's downtown core and left blocks of office towers and apartment buildings, including the courthouse, without power for days.
About 2,100 businesses and 5,000 residents were affected.
Investigators believe the fire's origin was in a faulty relay used to control street lights.
"The exact cause of the fire is difficult to determine due to the extent of damage caused by the fires," said Deputy Chief Ken Uzeloc.
"Investigators spent a fair amount of time sifting through the debris and reviewing the contents of the manhole. However, they were unable to determine why the relay failed or why the failure resulted in a fire."
A relay is an electronic circuit that uses a mercury sensor or a photo-electric sensor to switch street lights on and off. There are about 10,000 relays in Calgary that control about 90,000 street lights.
A city official said he's never seen anything like it.
"In the past, relay failures have resulted in street lights either not coming on or staying on in daylight hours," said Mac Logan, general manager of transportation.
Follow @BillGraveland on TwitterSuggest a correction