At least eight Americans have died in their rooms at resorts in the Dominican Republic under somewhat mysterious circumstances since the start of the year, according to news reports.

Some victims are said to have consumed alcohol before falling ill. Dozens more have reported getting sick while at island resorts, including 47 Jimmy Buffett fans who, People magazine reported this week, became violently ill during an April tour to the Dominican Republic.

Here’s what we know and don’t know about the mystery.

Who are the Americans who died?

Joseph Allen, 55, of Avenel, N.J., was found dead June 13 in a hotel room at the Terra Linda in Sousa after complaining to friends he did not feel well, WABC-TV reported.

Leyla Cox, 53, of Staten Island, N.Y., died June 11 while on a return visit to the Excellence resort in Punta Cana to celebrate her birthday, her son told NBC News. Will Cox said his family was trying to find out the circumstances surrounding his mother’s death.

Cynthia Ann Day, 49, and Nathaniel Edward Holmes, 63, of Maryland, were found dead in their room May 30 at the Grand Bahia Principe La Romana after they did not check out as planned.

Miranda Schaup-Werner, 41, of Allentown, fell ill and died May 25 after checking into the Luxury Bahia Principe Bouganville and taking a drink from the minibar.

John Corcoran, a retired roofing and siding executive in his 60s from Edgewater, N.J., and brother of Shark Tank star Barbara Corcoran, was found dead at the end of April in his room at an unidentified hotel in the Dominican Republic, Fox News reported.

Robert Bell Wallace, 67, of California, died April 14 after drinking a scotch from the minibar at the Hard Rock Hotel & Casino in Punta Cana and getting sick, his family told Fox News.

Jerry Curran, 78, a retiree living in Florida, died in January after falling ill following dinner and drinks the night of his arrival at Dreams Resort in Punta Cana, family members told WKYC-TV in Cleveland.

What were the causes of death?

No causes have been reported yet in the deaths of Allen, Cox, and Wallace, but Dominican Republic officials and the resorts have said the others died of natural causes.

The deaths of Day, Holmes and Schaup-Werner were attributed to enlarged hearts, internal bleeding, and pulmonary edema or fluid in the lungs.

Curran’s daughter, Kellie Brown, told WKYC that pulmonary edema was among the causes listed in his death.

Corcoran’s family said his cause of death was listed as a heart attack. Both Corcoran and Schaup-Werner had previous heart conditions, their families said.

Toxicology tests either are still pending or have not been officially reported in any of the cases.

Suspicions surrounding bootleg liquor, toxins

Investigators are looking into bootleg liquor as a possible cause in the tourist deaths, the New York Post reported.

In 2017, Dominican National Police dismantled five labs used for the manufacture of alcohol not safe for human consumption after five residents died from drinking tainted booze, according to the Post and NBC News.

Experts say another possibility is exposure to toxic chemicals like the pesticide that poisoned a Delaware family on a trip to the U.S. Virgin Islands in 2015. In that case, a man and his two sons experienced paralysis and were hospitalized for an extended period after they were sickened by methyl bromide, which was banned for indoor residential use in 1984 but had been used to treat the condo where they were staying.

What is the United States doing?

The Centers of Disease Control and the FBI are assisting in the investigations, officials said.

“The U.S. Embassy in Santo Domingo is actively working with the government of the Dominican Republic and the private sector at the highest levels to ensure that U.S. citizens are safe and feel safe while in the Dominican Republic,” the embassy said in a statement.

The State Department has not issued an advisory against traveling to the Dominican Republic but already warns tourist to use caution on the island because of the level of violent crime, the Washington Post reports. It also warns travelers not to drink alcohol alone or with new acquaintances, or to leave drinks unattended. U.S. citizens have been the targets of “date rape drugs” at parties and resorts, according to the warning.