FORT MCMURRAY, Alta. - Police in northern Alberta believe a hit and run semi was responsible for damaging an overpass on Highway 63.

They say the damage to the Hospital Street overpass in Fort McMurray has compromised its structural integrity and both lanes underneath the overpass have been closed.

RCMP say Alberta Transportation says one of the lanes is expected to be open Tuesday night, while three of the four lanes over the bridge have been reopened.

Police say they got a call about significant damage to the bridge Monday night just before 9 p.m.

They also got reports of concrete chunks falling onto the highway lanes.

RCMP say they believe a tractor trailer hauling an oversized load hit the bridge and fled.

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