"We can't say with certainty where this meat comes from because the documentation is so inadequate," Karin Cerenius of the control unit of the National Food Agency (Livsmedelsverket) told AFP.
The food security watchdog had been contacted by a consumer in Sundsvall in northern Sweden who was concerned over the size and colour of a beef steak she had bought from a local Ica Maxi grocery store.
The product was withdrawn from sale last week after tests showed it was 100 percent horsemeat.
The agency said inspections at Tallhöjden , based in Öxabäck in western Sweden, showed the firm had insufficient procedures in place for tracking "which products entered the company, which ones leave it, and how they are labelled."
The company handled both beef and horsemeat, Cerenius said.
Tallhöjden's head of sales, Johan Sahlin, said the meat had been ordered from another Swedish company that bought it from Poland, and insisted it had been labelled as beef.
"I bought beef fillet," he said.
The agency's report to prosecutors covers around 460 kilogrammes of meat, 300 of which remain at the Ica Maxi in Sundsvall, Cerenius told the TT news agency.
According to Tallhöjden, the mislabelled meat was also sold to a pizzeria.
The scandal of horsemeat being passed on as beef has engulfed a string of European countries with millions of ready meals pulled from supermarket shelves.
Around two weeks ago, food wholesaler Martin & Servera recalled around one tonne of roast beef that was labelled as pork from Poland, but which actually turned out to be horsemeat.