An early Friday morning fire in a commercial building in an industrial area in north Minneapolis is out, and investigators are now trying determine what caused it.

The 5 a.m. blaze that sent heavy smoke billowing into the air was confined to a domed structure on the 3700 block of N. Washington Avenue, but took several hours to extinguish, said Minneapolis Fire Department Assistant Chief Bryan Tyner.

The building near the Mississippi River was being used as a composting site and was filled with pallets of expired food and garbage, he said.

"The amount of stuff piled up made it difficult to get to the seat of the fire," Tyner said.

No one was injured.

Initial reports indicated that the building on fire housed a mushroom-growing operation that had recently closed. The building that burned is next to Mississippi Mushrooms, which closed last week amid building and fire code violations.

The building is located in the Upper Harbor Terminal. Plans call for the industrial area to be transformed into a riverfront park.