San Jose, Calif. ? Health officials have matched the deadly bacteria found in bagged spinach to samples of cattle manure found near a field where spinach was grown, providing a major clue to the source of an outbreak that killed three people and sickened nearly 200 last month.

The results from genetic testing could be the biggest breakthrough in 10 years of investigating outbreaks of illness from deadly E. coli bacteria that have been linked to leafy produce grown in the Salinas Valley, officials said Thursday. They said it may point the way toward preventing future episodes.

While eight earlier outbreaks have been traced to produce from the region, this is the first time that investigators have linked an episode to a specific source. But they are still working to determine how that particular E. coli bacteria may have migrated across a road that separates a cow pasture from the spinach field on the same ranch.