The fire at TreatCo, one of the nation's largest suppliers of natural dog treats, continued to burn late Friday, about 24 hours after it was first reported.
Fire investigators say they're not sure how long it will take to extinguish the flames.
"It's just a mess, with this fuel load and the way it's spread out," Fire Capt. Stuart Bevis said. "It's very difficult to deal with."
The blaze was first reported by someone passing by the plant at 2302 N. Broadway at about 8:15 p.m. on Thursday. The fire prompted three alarms before fire crews were initially able to put it out, officials said.
But another section of the plant erupted in flames shortly after 6:30 a.m. Friday. The second fire started in an area where boxes and packaging material are stored.
"Embers got into that area," Bevis said.
The fire originally started in the section of the plant used to dry pig ears.
Large amounts of boxes, packing material and animal fat were burning throughout the day Friday, Bevis said. Because the plant is essentially "a big concrete box," he said, it was retaining heat and making firefighting efforts challenging.
Conditions were so hot "it was literally just draining our fire crews," Bevis said.
Two people were arrested Thursday night after they shoved a firefighter and police officer in their attempts to enter the burning business, authorities said.
The two people — a 52-year-old man and 54-year-old woman — are co-owners of the company, according to TreatCo's website.
They were arrested shortly before 9 p.m. Thursday and booked into jail for battery of a law enforcement officer and interfering with firefighters, jail records show.
Police said the plant owner and his sister claimed they wanted to go inside to shut off utilities to the building.
That's a common reaction by owners of homes or businesses whose property is on fire, Bevis said.
"They want to do something," Bevis said. "They don't want to listen to reason."
TreatCo was the subject of an investigation by the Environmental Protection Agency in 2009, though results of that investigation have not been released.
TreatCo produces dog biscuits and cookies, jerky treats and animal body parts, like bones and pig ears, for pets.
The company processes more than 2 million pounds of product monthly from beef by-products it buys from Kansas and Texas slaughterhouses.