Florida health officials are investigating two more mysterious cases of Zika infection that do not appear to be related to travel, bringing the total to four.

The cases have raised the possibility that mosquitoes in the U.S. have begun to spread the virus. Florida officials say they are still looking into the cases and have not come to a conclusion.

The four cases are in neighboring Miami-Dade and Broward counties.

The cases fit a similar pattern seen when mosquito-borne clusters of two other tropical infections, dengue fever and chikungunya, occurred in Florida in the past, according to Tom Skinner, a spokesman for the U.S. Centers for Disease Control and Prevention.

“Evidence is mounting which points to local transmission via mosquitoes,” he said of the Florida cases.

The virus is mainly spread by mosquitoes, as well as sex. So far, the 1,400 infections reported in the U.S. have been linked to travel to countries with Zika outbreaks in Latin America or the Caribbean.

Zika causes only a mild illness in most people. But scientists recently confirmed that infection during pregnancy can lead to severe brain-related birth defects.

The tropical mosquito that spreads Zika and other viruses is also found in the southern U.S. Health officials have predicted that mosquitoes in the U.S. would begin spreading Zika this summer and have mobilized to keep it from spreading beyond isolated clusters.