Story Highlights• NEW: Theory: Chemical may have been added deliberately to raise protein level
• NEW: FDA: We've been working round the clock since we were told of recall
• Senate to hold hearings on FDA's handling of tainted pet food
• Recall expands to include dog biscuits, more Menu Foods products
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(CNN) -- Contaminants that led to a massive recall of pet food could have been added intentionally, according to one theory being considered by the Food and Drug Administration.
"Somebody may have added melamine to the wheat gluten in order to increase what appears to be the protein level," the FDA's Stephen Sundlof told CNN on Friday.
"Wheat gluten is a high-protein substance and by trying to artificially inflate the protein level, it could command a higher price. But that's just one theory at this point." (Watch theory about why anyone would deliberately adulterate pet food )
Sundlof said the agency is virtually sure the animal deaths linked to tainted pet food were caused by something that contaminated the wheat gluten, a normal ingredient of the food.
The FDA has found melamine, a component of fertilizers and plastic utensils, in the gluten, but that may not be the culprit, said Sundlof, director of the FDA's Center for Veterinary Medicine.
"Melamine is not very toxic as a chemical, so we're wondering why we are seeing the kinds of serious conditions, especially the kidney failure, that we're seeing in cats and dogs," he said.
"We are focusing on the melamine right now because we believe that, even if melamine is not the causative agent, it is somehow associated with the causative agent, so it serves as a marker," Sundlof said Thursday.
The recalled food has been linked to kidney failure in an undetermined number of dogs and cats. (Watch people whose pets died describe what happened )
The Senate's second-ranking Democrat announced Thursday the Senate will hold hearings on the FDA's handling of the recall.
"The FDA's response to this situation has been tragically slow," Senate Majority Whip Dick Durbin, D-Illinois, told reporters in Chicago. "Pet owners deserve answers. The uncertainty about what is safe to feed their pets has gone on far too long." (Watch Durbin call response to pet food recall 'a failure' )
Responding on Friday, Sundlof said, "We learned of the pet food recall from the company, Menu Foods, on the evening of March 15. We were in the plant on the morning of March 16, and since then we have had more than 400 people working on this issue virtually around the clock ... identifying the one company in China that produced this material."
The original recall included more than 60 million cans of "cuts and gravy-style" wet cat food and dog food made by Menu Foods.
Since then, the recall has broadened to include some pet foods produced by NestlÚ Purina PetCare Co., Del Monte Pet Products and Hill's Pet Nutrition.
And on Thursday, Sunshine Mills in Red Bay, Alabama, said 20 types of large dog biscuits are contaminated with potentially toxic wheat gluten from China. The dog-treat maker said it has received no reports of death or illness related to the products.
Company spokesman Conrad Pitts told CNN it purchased the tainted wheat gluten from The Scoular Co. of Minneapolis. Scoular officials said they bought the product from ChemNutra Inc. in Las Vegas, which recalled the tainted wheat gluten on Monday.
ChemNutra obtained it from Xuzhou Anying Biologic Technology Development Co. Ltd., a Chinese company. (Watch how the toxic food was traced to China )
The FDA has embargoed further imports of the Chinese company's wheat gluten, which it has determined was contaminated with melamine. Xuzhou Anying Biologic said it was astonished by the report but that it would cooperate with the U.S. investigation.
"We have never exported to the U.S. -- we are a trading company. We don't even know how we became implicated in this matter," Mao Lijun, the company's general manager, said Friday.
Asked if the company sold wheat gluten to another Chinese company that could have exported it to America, Lijun said he could not comment since the company was going through records to establish that.
The original recall announcement for Menu Foods covered products manufactured between December 3 and March 6. But on Thursday, the Ontario-based company widened it to include products dated back to November 8. (Details on recall)
Although no new brands were added on Thursday's amended list, Menu Foods added 20 varieties of pet food to the recall in response to ChemNutra's recall announcement. About 1 percent of the U.S. pet food market has been affected by the various recalls, the FDA said.
Official figure of 16 deaths expected to grow
Sundlof acknowledged that the official count of 16 pet deaths linked to the food will increase.
"We know that there have been a lot more animals affected by this, made ill and have died," he said. "Trying to put an estimate to it at this time is just not something we can do."
He said the agency is in the process of defining how to confirm suspect cases. The FDA has received 12,000 complaints during the three weeks since the recall was announced -- a number it would typically get over two years.
"Right now, our priority is still ensuring that all contaminated product is identified and removed from store shelves," Sundlof said.
The FDA said it has no evidence that any of the questionable wheat gluten has entered the human food supply.
CNN's Katy Byron, Susie Xu and Miriam Falco contributed to this report.
Veterinarian Michael Fusco checks Chloe, whose owner brought her in fearing the canine could have been fed tainted pet food.
MELAMINE-- White or colorless crystals used in the production of synthetic resins for plastic tableware and other products.
-- Melamine can cause mild irritation of the eyes, skin, nose and throat in humans.
-- Chemical linked to bladder cancer in male rats. Female rats suffered chronic inflammation of their kidneys.
Sources: OSHA, CDC
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