The FDA's real detective work may be just beginning. Having found many sources of contamination, investigators must now determine exactly how widespread the problem is and how it began.

The importer of the bad wheat gluten, ChemNutra Inc. of Las Vegas, contends that its Chinese manufacturer, Xuzhou Anying Biologic Technology Development Co., illicitly added melamine to the gluten in order to boost the measurable protein level and thus the price of the shipment. If so, the FDA may find itself pursuing criminal charges against the Chinese company.

Scores of pet food brands have now been recalled in the U.S. for fear that melamine-contaminated glutens were used in their manufacture. They include canned and dry dog food and dog biscuits that are made in places as widely scattered as Utah, Missouri and South Carolina.

The FDA is also examining imported vegetable proteins earmarked for human products such as pizza, protein bars and baby formula. That investigation, still in its early stages, hasn't uncovered any contaminated ingredients, but the agency, an FDA doctor said, wanted to "get ahead of the curve."

The melamine-laced food reached hogs because surplus pet food — crumbled and broken food bits rejected as unsuitable for dogs or cats — was sent to hog farms and turned into feed. The FDA says bulk shipments of feed were delivered to hog farmers in California, Utah, Ohio, Kansas, Oklahoma, New York, North Carolina and South Carolina. FDA officials said they were also concerned contaminated livestock feed may have been shipped to Missouri.

"It's absolutely a terrible nightmare story," said Eric Nelson, a Wisconsin feed specialist and president of the Association of American Feed Control Officers. "It just doesn't seem to get any better, and I'm sure it's not over."

Even as the tainted wheat gluten cases have multiplied, the FDA has learned of another problem: Chinese rice protein. U.S. importer Wilber-Ellis told the agency that a single bag of rice protein that it had imported tested positive for the presence of melamine. Wilber-Ellis imported the rice from Binzhou Futian Biology Technology Co. in China's Shandong Province. In the U.S., the protein went to five U.S. pet food makers in Utah, New York, Kansas and Missouri.

While the FDA has targeted select states for hog inspections, the pet food recall and the large number of sick cats and dogs have overwhelmed state agencies that often only investigate a dozen pet food complaints a year. The FDA says about 400 employees across the country are collecting pet food samples, monitoring the recalls' effectiveness and preparing complaints.

The investigation's progress in Illinois alone illustrates the problem.