California ? Kraft Foods Inc., the company whose testing led to the nationwide pistachio recall, said Friday it first heard there was salmonella in its trail mix in late 2007, but could not trace the possible source to tainted nuts from California until two weeks ago.

Workers at one of Kraft’s manufacturers in Illinois turned up a contaminated batch of fruits and nuts in December 2007. Then, in September of last year, another positive sample appeared.

Only after thousands of tests could the company pinpoint the source for the second positive test as California-based Setton Pistachio of Terra Bella, Inc., said Kraft spokeswoman Susan Davison.

Last week, the food products giant recalled or destroyed all suspect foods, and notified its suppliers and the Food and Drug Administration, which on Monday issued a sweeping national warning against eating the nuts.

“If we did detect salmonella, of course we would never ship our products,” Davison said. “We conducted extensive testing of all our food, and we were just unable to zero in until March that pistachios were the root cause.”