California health officials said Thursday that they still hoped to locate the source of the E. coli spinach contamination that has sickened at least 187 people in 26 states, killing one.

Officials stressed that the state’s farmers must do a much better job of adhering to safe agriculture practices to prevent future E. coli outbreaks. Those practices include making sure produce doesn’t come into contact with the E. coli bacteria from irrigation water, fertilizer, animal droppings or unclean human hands, the Associated Press reported.