A fire at a Portage dairy plant was believed to have started in a room where butter was located, according to firefighters.

The Portage Fire Department responded just after 9 p.m. Monday to Associated Milk Producers Inc. (AMPI) at 301 Brooks Street for a fire alarm.

Upon arrival, fire crews reported heavy smoke and fire from the roof of the dairy plant, according to the Portage Fire Department.

Firefighters stretched lines to the entrance door but were pushed back by the heat and smoke. Unable to access the structure, they used their monitor to fight the fire until Poynette and Kilbourne Fire arrived with their aerial apparatus.

The butter runoff and heavy smoke slowed access to the structure. After multiple hours with many crews fighting the blaze, the fire was contained and extinguished before it could spread past the firewalls and throughout the building. Crews were on scene just past 3 a.m. Tuesday.

Portage Fire Chief Troy Haase said the butter was the main obstacle to containing the fire.

“When we first tried to go up the stairs to that part that collapsed, this stuff, the butter was running down like 3 inches thick on the steps so our guys were up to their knees trying to go up the steps to get to the top and they’re trying to drag the hose line, the hose line got so full of butter they couldn’t hang onto it anymore,” Haase said.

Visible chunks of butter could still be seen at the plant on Tuesday.

The fire department noted that the Wisconsin Department of Natural Resources would visit the plant Tuesday to assess the butter runoff. Members of the Portage Hazmat Team had attempted to keep the runoff from going into storm sewers and into the canal.

The cause of the fire is under investigation.

A spokesperson with AMPI tells NBC15 that 30 employees were in the building at the time of the fire. They were all safely evacuated. Operations have been halted, no shipping will take place Tuesday and customers have been contacted.