The dates for reported illnesses ranged from July 12, 2019 to Sept. 8, 2019. CDC began the investigation of the outbreak on Sept. 17, 2019, at which time there were no further reported illnesses.
According to a Washington Post article, both the CDC and FDA "determined that romaine was the likely cause on October 2nd.", however, this was at a point when there had been no additional cases for several weeks.
https://www.fda.gov/news-events/fda-brief/fda-cdc-and-other-health-partners-investigated-outbreak-e-coli-o157h7-possibly-linked-romaine
FDA, CDC and other health partners investigated outbreak of E. coli O157:H7 possibly linked to romaine lettuce, outbreak appears to be over
October 31, 2019
https://www.fda.gov/news-events/fda-brief/fda-cdc-and-other-health-partners-investigated-outbreak-e-coli-o157h7-possibly-linked-romaine
FDA, CDC and other health partners investigated outbreak of E. coli O157:H7 possibly linked to romaine lettuce, outbreak appears to be over
October 31, 2019