Florida officials said Friday they have concluded that four Zika infections there were acquired from mosquitoes, the first confirmed cases of mosquito-borne Zika transmission in the continental United States.

Governor Rick Scott made the announcement in a news conference.

The four individuals, three men and one woman, are in Miami-Dade and Broward counties, which are adjacent. But health officials said Friday they think the specific location of the transmission is a one square mile north of downtown Miami.

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Health officials are continuing their investigation to see if other people have been infected and are trapping and testing mosquitoes in the area.

Federal health officials are assisting with the investigation and have called for blood centers in the two Florida counties to stop taking donations. The Food and Drug Administration has recommended that blood centers in adjacent counties also consider halting donations.

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But OneBlood, the nonprofit organization that manages most blood collection in the area, said Friday it was continuing with “business as usual” other than starting to test all of the blood it receives in Florida, Georgia, Alabama, and South Carolina for Zika virus.

Federal health officials are not calling for limiting travel to the Miami area, the Centers for Disease Control and Prevention said Friday. The CDC has for months said that pregnant women should avoid traveling to areas where Zika is actively spreading.

“As we have anticipated, Zika is now here,” the CDC’s director, Tom Frieden, said in a call with reporters Friday afternoon.

Frieden said the investigation has indicated that the four people contracted Zika in early July, started feeling sick about a week later, and were diagnosed within a few days. As they do with all cases, officials launched mosquito abatement efforts around those people’s homes and began to examine how they might have been infected.

The investigation honed in on the area north of Miami, where some of the individuals work, Frieden said.

“What’s concerning about this particular area is that more than one case was associated with it,” he said.

Frieden would not say whether the woman who is believed to have contracted the virus locally is pregnant, citing patient privacy.

Crews have stepped up their mosquito control efforts in the area, and Frieden said it would be concerning if additional people contracted Zika in the area after more aggressive mosquito abatement began.

Florida has seen more than 380 travel-related Zika cases so far, including more than 50 in pregnant women. There have been more than 1,650 cases in US states, almost all of which are travel-related. The virus can also be spread through sex.

The virus typically causes no symptoms or only a mild illness. But it can cause serious birth defects in fetuses when it infects pregnant women, including a condition called microcephaly in which the brain is underdeveloped and the head is abnormally small.

The Zika virus is primarily spread by Aedes aegypti mosquitoes, which are found throughout Florida and other states on the Gulf Coast. So far, Florida officials have not captured any mosquitoes that test positive for Zika, but even in places with widespread virus transmission, that might not happen. Experts liken it to finding the infectious needle in a haystack of virus-free mosquitoes.

The announcement Friday was in many ways expected since the state announced it was investigating four Zika cases in patients who had not traveled to Zika-affected areas. There have been 15 cases of sexually-acquired Zika in US states, but those have typically been tallied without specific announcements.

Infectious disease experts have been saying that Zika transmission was possible, even likely, in the continental United States as mosquito activity ramped up in the summer and as the number of people who were infected while traveling and returned home with the virus grew. The most likely route of local transmission is that a mosquito bites someone who has a travel-related Zika infection and then, if the mosquito becomes infectious itself, spreads the virus to people it bites later on.

It’s not known if all four suspected local cases of Zika are connected. The one square mile of Miami that officials are focusing on is obviously not a large area, but Aedes aegypti mosquitoes only fly a few hundred yards in their lifetimes.

Florida is one of the few states that has seen local transmission of other related mosquito-borne viruses in the recent past, such as dengue and chikungunya. In all those outbreaks, only a few people acquired the virus, which suggests this apparent local outbreak should not be widespread.

People in the continental United States are generally less exposed to mosquitoes than in other regions thanks to screened windows, the use of air conditioning, and better home construction. Densely packed urban areas in the rest of the Americas also make it easier for a mosquito to infect several people.