The strain of E.coli that caused 59 people across 15 states to become ill has been traced back to a reservoir on a farm in Santa Barbara County.

Water and sediment on the property of Adam Bros. Family Farms in Santa Maria tested positive for an almost identical strain of E. coli as the one found in people who have fallen ill. The Centers for Disease Control and Prevention still does not know how the reservoir became contaminated with the bacteria, and the finding still does not fully explain all of the illnesses that have been reported so far.

The agencies are continuing to investigate other possible sources, and the CDC still advises consumers not to eat romaine lettuce grown in California's Monterey, San Benito and Santa Barbara counties.

The CDC also advise that properly labeled romaine grown outside those counties and harvested after November 23 is okay to eat, as well as romaine grown in greenhouses or hydroponically.

Since the first cases of illness were reported in October, there have been 23 hospitalizations and two cases of hemolytic uremic syndrome due to the outbreak. The last reported illness was November 16.

CNN contributed to this report.