The US Food and Drug Administration requested Wednesday that Florida’s Miami-Dade County and Broward County temporarily stop accepting blood donations after four people in the area inexplicably came down with Zika infections.

The four cases were not immediately explained by travel to an area experiencing an outbreak of the mosquito-borne virus, nor from sex with an infected person—the two main ways US residents become infected. This has led some health officials to speculate, though not confirm, that local mosquitoes may be transmitting the virus to residents in those areas.

If local transmission is confirmed, the cases would represent the first homegrown outbreak of Zika in the continental US. It would also suggest that Zika infections, which are linked to birth defects in pregnant women but otherwise mild illnesses in adults, may be spreading undetected among Floridians. Thus residents could conceivably donate blood containing infectious viruses without knowing it.

To protect blood recipients from Zika, the FDA called on the two counties, as well as neighboring counties, to halt donations until health officials could start screening blood donors for Zika or set up technology that would inactivate Zika viruses in donated blood.

Infectious disease experts at the CDC and NIH have predicted small, contained pockets of local Zika transmission in the continental US, particularly in Florida. So far there have been 1,658 Zika cases, nearly all travel-related, in the US. Florida has reported 381 cases statewide.

“At this point we are not recommending a travel restriction to South Florida,” US Surgeon General Vivek Murthy told The Wall Street Journal Thursday. For now, residents of the affected areas, particularly pregnant women, are advised to take precautions to avoid being bitten by mosquitoes.