According to this article by Karen Roebuck of the Pittsburgh-Tribune Review, the FDA has found melamine, but not aminopterin, in the recalled pet food :
Because of a lack of scientific data, it is unclear what level of melamine would be harmful to cats and dogs, the sources said. An unknown number of cats and dogs nationwide have died or become ill with kidney failure...
Neither the FDA nor the independent lab tests found aminopterin. The agency is expected to announce its findings at a news conference at 10 a.m. today but is not commenting on New York's findings or that lab's methodology.
The FDA has not determined how melamine got into the pet food. Melamine is used as a fertilizer in Asia and has several commercial and industrial uses.
H/t to the bloggers and commenters over at Pet Connection.