Scientists are still unable to determine a definitive cause for reported canine illnesses associated with chicken jerky treats manufactured in China.

The Food and Drug Administration (FDA) has collected chicken jerky samples from all over the United States and tested them for a wide variety of substances.

In addition to some testing at FDA laboratories, the FDA says it is reaching out to private diagnostic labs to conduct analysis of the nutritional composition of 30 chicken jerky treat samples.

Results from March 2012 toxic metal analyses, which included tests for heavy metals, have again shown samples of chicken jerky products to be negative for toxic metals.

The FDA is submitting samples to multiple laboratories and testing for multiple culprits, including fatty acids, crude fiber, glycerol, protein, ash and moisture.

The FDA says it is testing nutritional composition of the treats in order to determine the concentration of glycerin in the various products, and is evaluating the ratios of the components in the sample treats.

"It is important to understand the composition of a product and its ingredients to determine where there might be a potential for problems to occur," the FDA posted on its website. "For example, during the pet food contamination investigation, FDA looked carefully at all the ingredients and it was later discovered that melamine was being used to raise the level of the protein in the products. Without a clear understanding of all the ingredients in a product, FDA cannot conduct a thorough analysis or investigation."

The FDA says that because no definitive cause has been identified, it has not taken action against any specific products or brands

The FDA continues to urge pet owners to use caution with regard to chicken jerky products.