MIAMI, United States (AFP) – Florida is investigating two new cases of Zika virus that may not involve people infected while traveling outside the United States, bringing the state’s total number of such cases to four, officials said Wednesday.

If any of the cases are confirmed, it would mark the first time that mosquitoes carrying the virus are known to be present in the continental United States.

Zika is spread via mosquitoes and by sexual contact. If pregnant women are infected they face a higher risk of bearing an infant with microcephaly, a birth defect that causes an abnormally small head.

“The department is expanding its ongoing investigations with two additional possible non-travel-related Zika virus cases in Miami-Dade and Broward counties,” the Florida Department of Health said in a statement.

“The investigations into the new cases will begin today, and door-to-door outreach and sample collection are ongoing in all cases,” it added.

The department urged residents and visitors to participate if asked for urine samples in the areas being investigated. The results would help determine the number of people affected.

Last week, Florida reported its first two cases of Zika virus in people who did not immediately appear to have any travel-related exposure — one in Miami-Dade County and one just to the north, in Broward County.

Those investigations are still ongoing, a spokeswoman told AFP.

The state has already seen 381 cases of Zika, all involving people who were infected while traveling to parts of the world where the virus is circulating.

For Zika to become a homegrown virus in the mainland United States, a mosquito would have to bite a Zika-infected person and then bite another person, passing on the virus.

Health officials have warned of possible localized Zika outbreaks in the United States, particularly since the virus has spread quickly throughout Central and South America and the Caribbean in the past two years.

However, mosquito control measures such as air conditioning, use of window and door screens and bug repellant are likely to prevent Zika from becoming established in the United States.