The FDA told ABC News Thursday that it is "still actively investigating the matter."
Previously, the FDA had sent a letter to Sen. Brown stating that veterinarians have tested samples and are awaiting results, but that "to date, scientists have not been able to determine a definitive cause for the reported illnesses.
The FDA issued its first warning about chicken jerky treats from China in 2007, in response to an initial wave of consumer complaints and a second in 2008. When the FDA issued a third complaint in November 2011, based on another increase in complaints, it asked consumers to report any other purported incidents directly to the agency. Since then, hundreds of dog owners have alleged that the treats have caused sickness or death in their pets.
The FDA also says on its website, "Some of the illnesses reported may be the result of causes other than eating chicken jerky."