The ongoing search for the source of a deadly E. coli outbreak in spinach has become a criminal investigation, as federal agents raid two California produce companies.

Agents from the Food and Drug Administration (FDA) and FBI have begun searching for evidence as part of a probe, sanctioned by the federal Food, Drug and Cosmetic Act, which makes the production or sale of "adulterated foods" a crime

In the wake of the ongoing probe into the source of a nationwide E. coli outbreak linked to tainted spinach, the Federal Bureau of Investigation raided two Salinas Valley produce companies.

On Wednesday, the FBI and the Food and Drug Administration's criminal investigations office issued search warrants for the Natural Selection Foods LLC plant in San Juan Bautista and Growers Express in Salinas to investigate if they violated food safety and environmental laws