Blood centers in two Florida counties have been instructed by federal authorities to stop collections till all units of blood are screened for the Zika virus amid investigations into four cases that could indicate the first instances of the infection being spread by mosquitoes on the United States mainland.

According to a statement on the U.S. Food and Drug Administration (FDA) website, blood centers in the Miami and Fort Lauderdale areas were asked to suspend collecting blood. Florida health officials are saying that the infections may not be linked to travel outside the U.S. However, there is no confirmation on how the virus spread in these cases.

No mosquitoes collected from the areas so far have tested positive for Zika, Jennifer Meale, spokeswoman for the state’s Department of Agriculture and Consumer Services, reportedly said, as investigators went door-to-door to collect samples and talk with residents in the affected areas.

The federal authorities recommended that neighboring counties take similar precautions “to help maintain the safety of the blood supply as soon as possible.” People who have visited South Florida in the past month have also been urged to defer donations.

As of now, the 1,400 infections reported in the U.S. have been linked to travel to countries in Latin America and the Caribbean with Zika outbreaks, according to the Associated Press (AP). Florida has 383 of these. In light of these instances, the FDA had previously advised U.S. blood banks to disallow people who recently travelled to areas that have Zika outbreaks from making donations.

OneBlood, Florida’s main supplier of blood, suspended collections in the areas of Miami-Dade and Broward counties earlier this week, according to a statement released Thursday. According to AP, OneBlood was working to comply with the federal authority’s “unanticipated” request. All its collections are set to be tested for the virus starting Friday.

Currently, there have been no reports of the virus in the country’s blood supply but officials say Zika transmission is possible through blood.

The Zika virus— spread primarily through the bite of an infected Aedes aegypti or Aedes albopictus mosquito — causes only a mild illness in most people, but the effects on pregnant women is more severe. Scientists have confirmed that infection during pregnancy can lead to severe brain-related birth defects.