California produce growers and processors worked to draw up new food-safety measures as government investigators trying to pinpoint the source of the deadly E. coli outbreak narrowed their search to three counties.

Western Growers, an industry group representing about 3,000 fruit and vegetable farmers in California and other states, planned to unveil a proposal Thursday for protecting produce from the bacteria that have killed one person and sickened at least 146 others across the country since last month.

"Generally, everything will be reviewed," said Tim Chelling, spokesman for Western Growers. "Health and human safety are our primary concerns."

Investigators found a contaminated bag of Dole baby spinach Wednesday at the New Mexico home of a person who fell ill. The spinach was packaged by Natural Selection Foods, a San Juan Bautista company that packages salad greens sold under dozens of brands.

After analyzing the strain of E. coli bacteria in the bag, investigators said they believe it probably originated in at least one of nine farms and several processing plants in California's Monterey, San Benito or Santa Clara counties.

E. coli is often spread by human or animal waste. Inspectors have been looking at the possibility that the germ was spread by contaminated irrigation water, workers relieving themselves in the fields, or some other means.

But Chelling said the effectiveness of any security measures taken by farmers and processors hinges on pinpointing the source of the outbreak.

"Absent that, you'll be applying a shotgun series of solutions that may or may not help," Chelling said.