VANCOUVER -- Firefighters confirmed an explosion under the Granville Street Bridge in downtown Vancouver Tuesday afternoon was accidental, but the exact cause is still being debated. Vancouver Fire and Rescue Services believe the blast was caused by electrical faults when underground cables failed. “The subsequent large electrical arc within a confined space resulted in the explosion, displacing concrete and damaging several vehicles,” wrote Capt. Jonathan Gormick in a statement. But BC Hydro investigators have another theory, and believe the blast was triggered by a build-up of gas. “That somehow ignited due to a manhole cover in the area being paved over with about a foot of concrete,” wrote Hydro spokesperson Mora Scott. “The source of the gas is still unknown. Until the investigation is complete, we won’t be able to confirm this. “