According to WBZTv.com  :

"Rodent poison has been found in pet food blamed for the deaths of at least 16 cats and dogs, a spokeswoman for the State Department of Agriculture and Markets said Friday.

Spokeswoman Jessica Chittenden would not identify the chemical or its source beyond saying it was a rodent poison.

State agriculture officials and Cornell University's Animal Health Diagnostic Center scheduled a news conference Friday afternoon to release laboratory findings from tests on the pet food conducted this week.

Neither agency would identify the poison, but one report identified it as aminopterin, which inhibits the growth of malignant cells and is used as a cancer drug. It also suppresses the immune system. In high doses, the chemical is highly toxic.

It is used to kill rats in some countries but is not registered for that use in the United States, according to the Environmental Protection Agency.

I talked about rat poison in a recent post being number 5 on the ASPCA top 10 poisons that harm pets.  Usually it is warfarin that is the major component of rodent poison which causes bleeding problems.  Hopefully they are able to identify the source of the problem and that pets lives can be saved.