Florida health officials concluded Friday that a small area of Miami-Dade County is likely experiencing Zika virus transmission due to local mosquitoes. The conclusion is based on a preliminary investigation into four recent cases of Zika virus that do not appear linked to travel.

“All the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago in several blocks in Miami,” Tom Frieden, director of the Centers for Disease Control and Prevention said in a statement. “We continue to recommend that everyone in areas where Aedes aegypti mosquitoes are present—and especially pregnant women—take steps to avoid mosquito bites.”

While no mosquitoes in the area have tested positive for the virus, the Florida Health Department released the neighborhood where the four cases are concentrated. Gov. Rick Scott’s office said this is the only area in the state where disease detectives are checking for local transmission.

“We learned today that four people in our state likely have the Zika virus as a result of a mosquito bite,” Gov. Scott said. “All four of these people live in Miami-Dade and Broward Counties, and the Florida Department of Health believes that active transmissions of this virus could be occurring in one small area in Miami.”

Scott added that health officials are “aggressively testing people” in the area to measure the extent of the outbreak. Health workers are heading door-to-door to collect urine samples and provide outreach. Blood banks are deferring donations from people living in the affected neighborhood until screening procedures are established.

Last month, Gov. Scott allocated $26.2 million for the state’s response to the Zika virus, which causes birth defects and the adult neurological disorder, Guillain-Barre syndrome. Approximately 80 percent of people infected by the virus do not show symptoms.

Close to 1,650 people have been diagnosed with Zika virus in the continental U.S., but until now, all of those cases have been linked to travel. Puerto Rico, in contrast, has been swarmed by more than 5,000 cases in 2016 with the bulk caused by local mosquitoes. Residents in 77 of 78 municipalities have tested positive for the virus.

“Puerto Rico is in the midst of a Zika epidemic. The virus is silently and rapidly spreading in Puerto Rico,” said Lyle Peterson, the incident manager for the CDC’s Zika Response.

So far, 672 pregnant women in Puerto Rico have shown evidence of Zika infection, and the CDC stated they “likely represent only a fraction of those who may be infected to date.” The public health agency also confirmed 21 cases of Guillain-Barre syndrome had occurred in the U.S. territory.

“This could lead to hundreds of infants being born with microcephaly or other birth defects in the coming year,” Peterson said. “We must do all we can to protect pregnant women from Zika and to prepare to care for infants born with microcephaly.”