SOUTH PORTLAND — The Maine Department of Transportation will investigate what caused a span on the northbound side of the Casco Bay Bridge — the side that allows traffic to enter Portland from South Portland — to become stuck Wednesday night.

The span became stuck and would not close around 7 p.m. but workers were able to get it to return to a level position about 20 minutes later.

Despite the fact that traffic was flowing smoothly again, MDOT spokesman Ted Talbot said an electrician will examine the bridge’s operating system to determine what caused the bridge span to malfunction.

Talbot said there are dozens of fuses — called limit switches — in the bridge, which are designed to detect problems such as improper alignment of decks.

One of those switches may have detected an issue and stopped the span from closing. Sometimes the buildup of ice and snow can cause decks to become misaligned.

The Casco Bay Bridge spans the Fore River and connects the cities of Portland and South Portland.