Fire broke out in a crude-oil processing unit at the Northern Tier Energy refinery Sunday morning, bringing firefighters from three departments, who brought it under control by midafternoon.

No one was injured in the fire at the St. Paul Park Refining Co. refinery — formerly the Marathon Oil refinery — and the unit, where crude oil is processed into asphalt, was the only part of the refinery that was shut down.

Firefighters were called about 11 a.m. and had the blaze under control by around 2 p.m., said Chief Kurk Lee of the St. Paul Park Fire Department.

The processing unit sits on a 25-foot-by-25-foot concrete pad and has a 50-foot-tall smokestack.

Lee said that an air leak ignited the fire and that investigators were working to discover the cause.

About six firefighters and six refinery fire-brigade workers monitored the unit until the fire was completely out about 3 p.m., officials said.

In addition to St. Paul Park, fire departments from Cottage Grove and Newport responded.