The first case of a mosquito giving Zika to someone in the US was always going to be a matter of when, not if. That “when” may be now. The Florida Department of Public Health announced late yesterday that it is investigating a case of non-travel related Zika in Miami-Dade County, at the very southeastern tip of the state.

A key part of the investigation will be testing if mosquitoes around the patient’s home carry Zika. The county had set out a type of mosquito trap, baited with carbon dioxide, that preferentially attracts the Zika-carrying mosquito species: Aedes egypti and Aedes albopictus. It’s now sent those mosquitoes to a lab at Florida Gulf Coast University to look for Zika virus. “We don’t have any results to report yet,” says Sharon Irsen, who co-leads the Florida Gulf Coast University lab with Scott Michael.

The university lab had been testing Miami-Dade’s mosquitoes for other tropical viruses, like dengue and yellow fever, for the past several years. Earlier this summer, they added Zika virus to their repertoire. The protocol is similar. “The mosquitoes are shipped in vials. We homogenize—basically mush up the mosquitoes—and extract the genome,” says Isern. Then they look for Zika’s genetic signature. The whole process takes about a week, but can go faster if necessary.

The Florida Department of Public Health did not specifically rule out sexual transmission of Zika and did not provide any additional details about the case. But sexual transmission is generally easier to prove, as long as you can test the partner. When Dallas found the first case of sexual transmission in the US back in February, they came out and said it. This week also saw a mysterious Zika case in Utah, which does not appear to be associated with mosquitoes, travel, or sexual contact.

If the Zika case in Miami-Dade is indeed traced back to local mosquitoes, it could herald more endemic infections to come. “It’s not a small story,” says Michael. “The mosquitoes that can transmit are all up and down the east coast and the Gulf coast.” Still, it is unlikely to ignite a full-blown epidemic like that seen in Brazil. Take the case of dengue, a similar virus spread by the same mosquito species that spread Zika. Florida has seen sporadic breakouts of dengue, but public health officials were able to contain them by containing the insects.

Zika has made residents more wary of mosquitoes. Miami-Dade county’s mosquito control has responded to 5,549 mosquito control requests from residents between October and July 7, compared to 4,408 for all of the previous year, according to county spokesperson Gayle Love. When mosquito control gets these requests, officials go out to drain any small pools of standing water in yards and spray bigger ones with larvicide. They’ll also check a few neighboring yards, because the Aedes mosquitoes can only fly so far.

This has been, and will continue to be, the core strategy behind containing Zika. The best way to prevent mosquitoes from transmitting Zika is to prevent mosquitoes in the first place.