Nearly 600 pets have died and thousands more have been sickened after eating jerky treats produced in China — and the Food and Drug Administration has no idea why.

The FDA on Tuesday announced that since 2007 it had logged around 3,600 reports of dogs and 10 reports of cats becoming ill after consuming Chinese-made treats made primarily with chicken and duck. Of those cases, 580 animals died.

Despite the growing number of reports, though, the FDA has been unable to identify the root cause. That's not for a lack of effort. The agency said it had conducted more than 1,200 tests of Chinese jerky in recent years, visited the treats' manufacturers, and reached out to foreign experts and leading academics for help.

Still, the spate of illnesses remains "one of the most elusive and mysterious outbreaks we've encountered," Bernadette Dunham, head of the FDA's Center for Veterinary Medicine, said in a statement.

The FDA has been raising red flags about the safety of Chinese-produced snacks for some time.

Back in 2007, the FDA launched an investigation into dog treats pulled from Walmart stores. And in January, two of the largest retailers of pet jerky — Nestle Purina Petcare and Del Monte Corp. — pulled select products from shelves after they were found to contain residual traces of banned antibiotics.

However, the FDA said at the time that there was "no evidence that raises health concerns," and that the products were "highly unlikely" to be related to the mounting death reports. (The antibiotics in question are outlawed in the U.S., but widely used abroad, including in the European Union.) And while the rate of illnesses has fallen since that recall, the FDA theorizes that the drop was likely the result of a decreased demand for treats overall, and not the result of deadly meat leaving the market.

The rise in pet illnesses correlates to a massive spike in the volume of pet food imported from China. In 2003, the U.S. imported 1.1 million pounds of cat and dog food from China; that figure leapt to 85.8 million pounds in 2011.

Several federal lawmakers last year asked the FDA to issue a blanket recall of Chinese jerky treats, questioning why it had yet to do so despite thousands of reported illnesses. Former Rep. Dennis Kucinich (D-Ohio) labeled the products "death treats," while Rep. Jerry McNerney (D-Calif.) wrote to the Chinese government asking it to consider a moratorium on production until the FDA could determine whether the products "contain tainted material."

Given the booming business, though, China's government declined the request, instead casting blame on the FDA.

"From the perspective of the Chinese side, there might be something wrong with the FDA's investigation guidance," Beijing wrote.

For its part, the FDA has said such a recall would be impractical given the government's limited understanding of the cause of the illnesses. Instead, the agency has now called on veterinarians and pet owners to help them out and report any new suspected cases as soon as possible.