A compilation of data from this secondary investigation demonstrated that most patients had an onset of symptoms within a fairly narrow window of time, with the highest rate of onset occurring on May 17. Taking into account the incubation period of E. coli infection, the medical team determined that most patients had been exposed to the pathogen between May 12 and May 14. Investigators had less luck trying to pinpoint a physical source of the infection; when affected patients' residences were mapped, their distribution across a map of Walkerton showed no immediately apparent pattern, with affected populations present in all areas served by Walkerton's water system. By that evening, public health investigators had concluded based on patient interviews and demographic mapping that the water system was the most likely vehicle of infection, despite their not being aware of the corroborating documents that John Earl had collected. They began notifying branch offices of the BGOSHU of the outbreak and its cause. Events of May 23 and 24 The Bruce-Grey-Owen-Sound Health Unit (BGOSHU)'s May 22 presumptive conclusion about Walkerton's water was confirmed at 8:45 the following morning, when the London Regional Public Health Laboratory reported testing results on the water Schmidt had collected on May 21 and 22: both E. coli and fecal coliform results had come back positive on the May 21 samples, and though testing was not complete on the May 22 samples, evidence of coliform growth was already apparent. John Earl (the Ministry of the Environment (MOE) environmental officer) returned to the Walkerton Public Utilities Commission (PUC) that morning to continue his investigation. Stan Koebel, the PUC manager, gave him the Well 7-related documents that he and Frank Koebel, the PUC foreman, had falsified, and Earl began to analyse the documentation he had collected over the past few days. He noted apparent irregularities in the pumping histories of all three wells, as well as unlikely chlorination levels recorded for Well 5. He communicated these irregularities to the MOE's Philip Bye, but neither man reported them outside of his own organization. At 9:45 on the morning of May 23, David Patterson (the Assistant Director of Health Protection at BGOSHU) contacted Stan Koebel at the Walkerton PUC and informed him of the May 21 test results. Koebel, realizing that his attempts to clean the system beginning May 19 had not been sufficient, confessed to Patterson that he had been aware of