California ? Despite an unprecedented investigation over the last six months, federal and state health officials said Tuesday that it is unlikely they will ever pinpoint the exact cause of bacterial contamination that caused a nationwide outbreak of illness from tainted spinach last year.

The disclosure, just weeks before a final report on the investigation is expected to be released, came at a hearing where representatives from consumer groups and a national trade association for the produce industry called on the U.S. Food and Drug Administration to set mandatory rules to assure that fruit and vegetables are safe to eat.

FDA and state health officials, however, said they favor voluntary guidelines and industry self-policing, including a set of standards that California growers are expected to adopt next week. The government officials said they would not rule out mandatory regulation in the future.

Growing and packing practices need to improve, said Dr. David Acheson, the chief medical officer for the FDA’s food safety office.