The Food and Drug Administration's efforts to combat food-borne illness are hampered by infrequent inspections, not enough staff and the failure to implement a program devoted to the safety of fresh produce, according to congressional investigators.

The Government Accountability Office draft report obtained Thursday by The Associated Press also said that only 1 percent of produce imported into the U.S. is inspected, and that the practice of mixing produce from several sources makes it hard to trace contamination.

The findings, to be released Friday, outraged Sen. Barbara Boxer, a California Democrat who along with Massachusetts Sen. Ted Kennedy called for the investigation after the 2006 E. coli contamination in bagged spinach killed three people, sickened 200 others and cost the leafy greens industry $86 million.

"This report paints a frightening picture of the FDA's fresh produce safety efforts," Boxer said. It "should serve as a wake up call to do more to protect the nation's food supply."

A spokesman for the FDA would not comment until the report was released.

The report said that inspections at produce-processing facilities are rare and that when problems are discovered, the FDA relies on the industry to correct them without oversight or follow-up.

From 2000 to 2007, the FDA detected food safety problems at more than 40 percent of the 2,002 plants inspected, yet half of those plants were inspected only once during the period. The plants with food-safety problems received only warning letters from the FDA, and even those ended in 2005.

"The agency seized no fresh produce, sought no injunctions and prosecuted no firms," investigators said.

The GAO said some of the FDA's problems can be attributed, in part, to funding.

As the amount of fresh produce imported into the U.S. has grown, the FDA's inflation-adjusted budget has remained stable, which has forced the agency to absorb cost-of-living increases for staff members, the report said.

The result is early retirements and a decision not to fill some positions that have reduced food safety staffing by 17 percent, including 800 scientists, inspectors and other food-safety staff, the investigation showed.

The loss of key scientists and the agency's failure to replace them means food safety guidelines have not been updated and the industry is relying on its own rules for product safety.

The lack of specific knowledge, the report says, has prevented the FDA from developing "robust, science-based regulations and risk assessments that quantify the relative risks of consuming different types of produce."