MIAMI — Federal officials want to suspend blood donations in two Florida counties where officials are investigating four cases of Zika infection that may be the first spread by mosquitoes on the United States mainland.

Blood centers in the Miami and Fort Lauderdale areas were being asked to stop collecting blood until they could properly screen each unit for the Zika virus, according to a statement on the Food and Drug Administration website.

Florida health officials said the four infections in Miami-Dade and Broward Counties might not be linked to travel outside the United States, but they had not confirmed how the virus spread.

Investigators were going door to door to talk with residents and collect samples. No mosquitoes collected in the affected areas had tested positive for the Zika virus, said Jennifer Meale, a spokeswoman for the state’s Agriculture and Consumer Services Department.