BLOOMINGTON, Minn. — The Minnesota Department of Commerce said Thursday that biodiesel was not what caused a dozen school buses to malfunction in frigid temperatures the previous week.

“An independent investigation confirmed what we believed last week: When it gets to 20 degrees below zero in the Midwest, diesel engines have trouble operating unless they are properly maintained — whether or not they are using a biodiesel blend,” said Bill Walsh, communications director for the Minnesota Department of Commerce.

On Jan. 15, Bloomington Bus Co. began experiencing filter plugging on some of its school buses, which run on a B2 biodiesel blend. Twelve of the company’s 109 buses would start but only run for a short amount of time before the filters plugged and the buses stalled.

An investigation conducted by MEG Corp. and paid for by the fuel supplier, Yokum Oil, analyzed the filters and found that components of diesel caused the malfunctions.

“We found that whatever was plugging the filters was not biodiesel, but a substance found in petroleum,” the MEG Corp. report said.

B2 is 98-percent petroleum diesel blended with 2-percent biodiesel. Minnesota requires diesel sold in the state to be at least a B2 blend.

No other school bus operators in the area reported similar problems.