NEW YORK — U.S. health officials disclosed another food poisoning outbreak linked to romaine lettuce, but they said it appears to be over.

The disclosure late Thursday comes after the produce industry said it was stepping up safety measures following a series of outbreaks, including one last year that sickened more than 200 people and killed five. It’s not clear why romaine keeps sickening people, but experts note the difficulty of eliminating risk posed by raw vegetables grown in open fields.

The U.S. Food and Drug Administration said 23 people were sickened between July 12 and Sept. 8. No deaths were reported. As with previous outbreaks, the agency said it was unable to determine how the romaine became contaminated.

The Centers for Disease Control and Prevention said by Sept. 19 leafy greens were suspected for the cluster of E. coli illnesses, and that romaine was determined as the likely source Oct. 2.