The woman who set off the Zika scare in Florida doesn’t have a clear connection to the neighborhood where the outbreak is believed to be concentrated. She hasn’t traveled to a country where Zika is circulating and she hasn’t had sex with anyone likely to be infected.

These confounding facts are also laced with potential danger. The woman in her early 20s is pregnant.

The woman is one of 21 cases health officials are grappling with in their efforts to understand and contain the first known mosquito-borne Zika outbreak in the continental U.S., according to an internal report on the investigation for health officials reviewed by The Wall Street Journal. She is also the first known pregnant woman likely infected in the U.S. by a mosquito bite, rather than from travel or sexual intercourse.

Referred to as Miami-Dade #1 for the county she lives in, the woman isn’t known to be connected to the “warning zone” of about a square mile in the neighborhood of Wynwood, just north of downtown Miami, that is now the focus of Florida and federal health investigators.

Two other cases in the report, a man from adjacent Broward County and a man from Miami-Dade, also don’t have a clear connection to the area, showing that significant questions remain about how and where patients are being infected with Zika, and how widespread the transmission may be.
Investigators were led to Wynwood, a neighborhood full of hip restaurants and art galleries but also vacant storefronts and residential blocks, by two other cases: infected men who work in businesses about 400 feet from each other there, according to the report.

One, a man in his late 20s known as Broward #2, has a job at a business that does work in Brazil and has several employees who travel there frequently, according to the report. The report didn’t name the business or say what kind of work it does.

The patient recently had a houseguest from Brazil, where an explosive epidemic last year led to nearly 166,000 suspected infections and has been tied to birth defects in hundreds of newborns. Fourteen of the roughly 30 employees who work at the business in Wynwood reported they had typical Zika symptoms such as fever, rash, joint pain or conjunctivitis from early June to mid-July, according to the report.

The other infected man, a 26-year-old known as Miami-Dade #2, works at a business nearby with 11 employees. He and a customer were the only ones to report Zika symptoms, according to the report.

The Florida Department of Health said it had identified 17 positive cases so far from the roughly square-mile area, including four new cases announced on Tuesday. Most of these cases have been in the area of the businesses, according to the report and officials familiar with the investigation.

One patient who works in the area said he visited the emergency room with a fever and a rash on Aug. 1; doctors suspected Zika and drew blood for testing. On Tuesday it was confirmed that he was infected with Zika, likely from a mosquito.

On Monday, a new case was reported in Palm Beach County. The case expanded the outbreak to a third Florida county, north of Broward and Miami-Dade counties, where the first cases were identified. Florida Department of Health officials have ruled out sexual contact as the source of the latest infection, a spokeswoman said. The agency is conducting a door-to-door investigation in Palm Beach County to see if there are additional Zika cases there.

The gender of the patient wasn’t reported. Of the 21 known cases, 17 are men, according to the Florida Department of Health.

The CDC warned pregnant women on Aug. 1 to avoid the roughly square-mile area in Wynwood around the central site of the two businesses—the first such warning of its kind on the U.S. mainland.

The Florida Department of Health said it believes that active transmission of Zika is occurring only within the roughly square-mile area, but that it is investigating elsewhere and will notify the public if it finds wider active transmission. The CDC has said that single, isolated cases are to be expected when local transmission by mosquitoes occurs.

The CDC referred questions about individual patient details to the Florida Department of Health, which said it wouldn’t confirm any.

Most of the 1,825 confirmed cases of Zika in the U.S. mainland are related to travel to Zika-infected areas, the Centers for Disease Control and Prevention said last week.

The report and interviews with officials are a reminder how difficult it is to stop a mosquito-borne outbreak. Despite preparations for Zika in the U.S., it took weeks from when the first cases were likely infected by mosquitoes for authorities to determine where transmission was likely occurring. Efforts to kill mosquitoes initially had limited success.

The unnamed pregnant woman was the first patient to raise alarm bells among authorities.

The young woman was 23 weeks pregnant, unemployed and living with her boyfriend in a rooming house with no air conditioning or window screens. She went to an emergency room on July 7 with symptoms of Zika after feeling ill for three days.

The virus is dangerous for pregnant women, because it can damage the developing brain of a fetus. The report said the patient was given an ultrasound a week or two after the Zika infection, and her baby appeared to be growing normally.

The second case that emerged, known as Broward #1, a 44-year-old man who went to the doctor a day after the pregnant woman, lives about 14 miles from her and had no close geographic contact, according to the report.

It wasn’t clear from the first two cases where mosquitoes might be spreading Zika. The Florida Department of Health went door to door, asking people about possible Zika exposure and if they would give urine samples for testing, said Celeste Philip, Florida’s surgeon general and health secretary, and others involved in the effort. Investigators tested 124 people in Miami-Dade and Broward connected to the pregnant woman and the man from Broward, according to the report and investigators, and found no Zika cases.

A Florida Department of Health spokeswoman said the first cases had each been deemed to be an “isolated incident” not tied to the Wynwood area.

The next two cases gave investigators a connection to Wynwood through their workplaces, according to the report. Those men, Broward #2 and Miami-Dade #2, had fallen ill a few days after the pregnant woman, complaining of fever, rash and aching joints, but said they hadn’t traveled, according to the report.

Another infected person—a 33-year-old man called Miami-Dade #3, who fell ill July 9—lives about 3 miles south of the two Wynwood businesses, according to the report. The document didn’t give any information linking the case to the Wynwood area. The Florida Department of Health said it had gone door-to-door to interview and test people in possible areas where the individual outside of Wynwood could have been infected. It said Tuesday its investigation continues.

After the two cases alerted investigators to the link to Wynwood in late July, teams of public-health workers set about canvassing for possible additional infections.

The focus was on an area radiating 500 feet out from one or both of the affected businesses, reflecting the average flight range of the Aedes aegypti mosquito, which transmits the virus.

Zika is a hard virus to track because four out of five people who are infected aren’t aware. They don’t develop symptoms or have only mild symptoms.

Miami Police Commander Jose Rodriguez accompanied health investigators in Wynwood to reassure residents, who were often wary of unfamiliar faces at the door. Usually “we tell people don’t open the door to strangers,” he said.

Officials collected urine samples, distributed fliers on how to prevent an infection and scoured properties for standing water, said Mr. Rodriguez. Orange and blue fliers hanging on doorknobs in Wynwood urged residents “Don’t forget to drain and cover.” Use repellent and put screens on open windows and doors to “protect yourself from mosquito bites and the diseases they may carry,” fliers said.

A mosquito can bite an infected person, for example someone who is traveling from a place where the virus is circulating, and then transmit the virus to other people it bites, triggering a local outbreak.

The CDC said it doesn’t expect transmission of Zika in the continental U.S. to become widespread, noting the Aedes aegypti mosquito’s short flight range.

Enough Americans live in air-conditioned, mosquito-free homes that sit apart from one another that it isn’t easy for the bugs to transmit disease to many people or for people to carry it to many new spots, something much easier in the crowded urban neighborhoods of the tropics, the CDC said.

It probably will never be known for sure how mosquitoes began transmitting Zika in Miami, partly because most people who are infected don’t know it, said CDC Director Tom Frieden. “It’s unlikely that we’ll ever know exactly who brought it in, where they brought it in from,” he said.

Miami-Dade County officials said they have found no Zika-infected mosquitoes. Miami-Dade County mosquito-control workers are aggressively exterminating the insects, spraying insecticide from airplanes.

They did that after earlier efforts on the ground didn’t significantly reduce the population of Aedes aegypti—a nasty pest that breeds in tiny pools of water in bottle caps or trash, hides in dark recesses and is hard to get rid of.