Calling it "a nightmare" of a fire, Wichita firefighters continue to battle a four day-old blaze at the  TreatCo dog treat factory, 2300 N. Broadway.

Thick smoke continues to pour from the upper floors of a storage building where boxes of greasy dog treats sit on wooden pallets, fire investigator Stuart Bevis said.

"Basically you have a huge concrete box full of fuel and it just makes it a nightmare to try to take care of, to ventilate to extinguish, to access," he said.

The cause remains a mystery until the factory is safe enough for investigators to enter.  There are questions about the structural integrity of the large concrete building after days of fire.  Because of the potential for danger, fire crews are not being allowed inside the building and must fight the flames by dousing them with water or foam from a distance, Bevis said.

This isn't the first time TreatCo has had problems.  Factfinder 12 Investigators learned the company, which uses meat byproducts to manufacture dog biscuits, jerky and other pet treats, has been investigated by federal environmental and immigration officials.

In 1998, the Immigration and Naturalization Service (INS) served a warrant to inspect the TreatCo property after officers suspected the company was housing and employing illegal immigrants.  INS stated Employment Eligibility Verification forms had been falsified and that TreatCo workers "utilized fraudulent documentation or social security numbers relating to other individuals to obtain employment".  During the inspection, officials took eight suspected undocumented immigrants into custody, court documents revealed.

Since 2001, TreatCo has received at least 30 environmental violations from the City of Wichita, most of them for breaking the Clean Water Act.  The company was also found to be processing wastewater for other companies, government documents show.  TreatCo remains the subject of an investigation by the EPA's criminal division and the U.S. Attorney's office.