Community transmission of the novel coronavirus in the United States began as early as late January or early February—weeks before the first confirmed case of non-travel-related infection on Feb. 26, a new report from the Centers for Disease Control and Prevention shows. The analysis by the CDC involved four separate lines of evidence, including virus surveillance, phylogenetic analysis, and retrospectively identified cases. Previous reporting and genetic analyses of the virus had suggested that cases likely occurred as early as late January, but the CDC report on Friday was the first official confirmation from the federal government. The first community spread case was possibly related to a Washington state-based patient who traveled from Wuhan, China, and developed symptoms on Jan. 19. That cluster likely then spread “throughout the Seattle metropolitan area and possibly elsewhere,” according to the report.

To be clear, the prevalence of pre-symptomatic and even asymptomatic transmission of the virus mean that authorities may never be completely certain about where and when the first case of community transmission originated.