The Zika virus appears to have entered the mosquito population of the U.S. mainland: Four cases in Florida are highly likely to be caused by infected mosquitoes, the Florida Health Department announced today (July 29).

Officials haven’t yet trapped any mosquitoes that tested positive for the virus, but they believe the infected insects are limited to a one square mile area in Miami-Dade County.

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To further the investigation, the health department is going door to door in the area to collect urine samples and notify residents. This will bring the department a step closer to understanding the gravity of the situation.

The state has already seen 386 travel-related Zika cases with 55 cases involving pregnant women.