More than 2 million Americans visit the Dominican Republic every year, making up about a third of the country’s tourists. Six have died in the last year during their visits. Because of the seeming similarities among their deaths, their family members have suggested that they are connected and have raised suspicions about the resorts where they died. Here’s what we know, and don’t know, about the circumstances.

Who has died, and how?

Yvette Monique Sport, 51, died in June 2018 of a heart attack. Her sister, Felecia Nieves, has said that Sport had a drink from the minibar in her room at a Bahia Príncipe resort, one of a number on the island, then went to sleep and never woke up.

In July 2018, David Harrison, 45, died at the Hard Rock Hotel & Casino Punta Cana. Harrison died of a heart attack, and The Washington Post reported that his death certificate also listed “pulmonary edema, an accumulation of fluid in the lungs that can cause respiratory failure, and atherosclerosis” as causes of death. He and his wife were in the Dominican Republic for their wedding anniversary with their son.

In April of this year, Robert Wallace became sick at the Hard Rock Hotel & Casino Punta Cana, where he was attending a wedding, and died. The 67-year-old’s family said that he became ill after drinking scotch from the minibar in the hotel.

Miranda Schaup-Werner, 41, of Allentown, Pennsylvania, was celebrating her 10th wedding anniversary when she died at the Luxury Bahia Príncipe Bouganville, on May 25 of this year of a heart attack. She had been at the resort for less than 24 hours.

A few days later, Nathaniel Edward Holmes, 63, and Cynthia Ann Day, 49, from Prince George’s County, Maryland, were found dead in their room at the Grand Bahia Príncipe La Romana. The two had recently become engaged. An autopsy found that the couple had respiratory failure and pulmonary edema.

Are the hotels connected?

Four of the dead were staying at Bahia Príncipe resorts, which are part of a group of 14 hotels in the Dominican Republic that are popular among tourists because they are all-inclusive. The Luxury Bahia Príncipe Bouganville, where Schaup-Werner died, is less than a five-minute walk away from the Grand Bahia Príncipe La Romana, where Holmes and Day died. Both are near the town of San Pedro De Macoris. The Hard Rock is across the island from the other two hotels in Punta Cana. It is not known which Bahia Príncipe resort Sport was staying in.

What are the hotels saying?

In a statement Friday, Bahia Principe said reports of the deaths had been inaccurate and that the hotel was committed to “collaborating completely with the authorities and hope for a prompt resolution of their inquiries and actions.”

Hard Rock Hotels & Casinos said in a statement Tuesday evening that it is waiting for official reports about the deaths and is “deeply saddened by these two unfortunate incidents, and we extend our sincerest sympathy to the families of Mr. Harrison and Mr. Wallace.”

What are Dominican officials doing?

The Dominican attorney general’s office and the national police are investigating the deaths, but tourism officials have been downplaying them. The tourism minister, Francisco Javier García, said last week that in the last five years, more than 30 million tourists have visited the country, and that these deaths are “isolated incidents” and the island is safe for tourists.

“These are situations that can occur in any country, in any hotel in the world,” he said. “It’s regrettable, but sometimes it happens.”

The tourism ministry said last week that hotels had 60 days to install security cameras.

What are U.S. officials saying?

In a statement issued Tuesday evening, the U.S. State Department said that “Dominican authorities have asked for FBI assistance for further toxicology analysis,” and it could take up to a month to receive the results. A spokeswoman for the Centers for Disease Control and Prevention said that the organization had not received a request for assistance from the Dominican Republic relating to these deaths.

Are there any theories as to what might be causing the deaths?

Tom Inglesby, director of the Johns Hopkins Center for Health Security, said in a phone interview that the symptoms that have been reported, like pulmonary edema, bleeding and vomiting blood, are “consistent with poisoning,” perhaps accidental. But until toxicology reports are available, he said, it is difficult and too soon to definitively say what caused the visitors’ deaths.

“It’s rare for travelers to die of unknown causes like this, and to have a high number of them in a relatively short period of time is alarming, shocking, sad,” Inglesby said. “It’s something that investigators should be able to get to the bottom of.”

The fact that toxicology reports have not been released or completed is “unconscionable and inexplicable,” he said.

Have there been other incidents?

Two couples have come forward to say they fell ill while staying at one of the Bahia Príncipe resorts where tourists have died.

In January 2018, Doug Hand, 40, and his wife Susie Lauterborn, 38, were staying at the Grand Bahia Príncipe La Romana when, he said in a phone interview, they got sick with fevers, nausea, cold sweats, diarrhea and fatigue.

Hand said that he didn’t drink alcohol on the trip, but he did notice a “moldy, mildewy smell like the AC or filter hadn’t been cleaned.”

When Hand told an employee in the hotel’s lobby that his wife was sick, the employee gave him directions to a doctor, but seemed more focused on ensuring the couple attended a meeting about buying time shares, Hand said.

Kaylynn Knull, 29, and Tom Schwander, 33, are suing the resort chain for $1 million, their lawyer told The Times, because the Colorado-based couple became violently ill during their stay at the Grand Bahia Príncipe La Romana last summer. Knull got a persistent headache and was sweating and drooling profusely, the lawyer, David Columna, said. She also had blurry vision, nausea and diarrhea, she told CNN, and family doctors determined the couple had been exposed to organophosphates, a class of insecticides.

“The hotel did nothing,” said Columna, who is representing Knull and Schwander in the Dominican Republic. The couple, he said, “spent the night inhaling the chemical, and they are still having side effects of the intoxication, and the hotel hasn’t given us any idea of what happened.”

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