“I am writing today regarding the unexpected and highly suspicious deaths of at least nine American tourists in the Dominican Republic,” Pallone, who addressed his letter to Secretary of State Mike Pompeo and FBI Director Christopher Wray, wrote Wednesday. “The circumstances surrounding the untimely deaths of nine Americans is heartbreaking, and I ask that you immediately take steps to update the bereaved families and ensure they are given all information on the cause of their loved one’s death as the investigation continues.”

Since June 2018, at least nine Americans have died in the Dominican Republic, many of whom were reported to be in good health before they succumbed to symptoms ranging from pulmonary edema and bleeding to respiratory failure and heart attack. Joseph Allen , a 53-year-old from Avenel, New Jersey, was the latest traveler to die while on vacation in the country. He was found dead in his hotel room last week.

Rep. Frank Pallone (D-N.J.) issued calls for the State Department to reevaluate its travel advisories for Americans heading to the Dominican Republic on Wednesday, saying he was troubled by a spate of “untimely deaths” among tourists in the popular Caribbean destination, particularly after a man from his own congressional district was the latest person to die.

I'm extremely saddened by the deaths of 9 Americans in the #DominicanRepublic . One of the victims was from my district in New Jersey. Today, I wrote the @StateDept & @FBI calling for a reassessment of the travel advisory. The families deserve timely updates on the investigation. pic.twitter.com/jpTZVDfEJh

Many others have reported falling ill while in the Dominican Republic, including 47 people on a Jimmy Buffett fan tour who were staying at a resort in the country. They reported violent sickness, including “crippling vomiting,” diarrhea and headaches.

But it’s still unclear if anything strange is behind the events. The FBI is working with local authorities to investigate any similarities behind the deaths and the State Department said it could take up a month to receive results from any toxicology analyses.

“The safety and security of U.S. citizens that live in, work in, and visit the Dominican Republic remains our highest priority. These incidents are tragic and we offer our deepest condolences to those personally impacted,” Robin Bernstein, the U.S. ambassador to the Dominican Republic, said earlier this month. The embassy in Santo Domingo also said local authorities haven’t yet established any connection between the incidents.

Officials in the Dominican Republic itself have moved to downplay the deaths, and the country’s tourism minister, Francisco Javier García, said last week that they merely represented “isolated incidents.”

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

The Department of State issued a Level 2 travel advisory for the Dominican Republic in April, warning travelers to exercise increased caution due to crime. That warning predated the spate of deaths and illnesses, however.

More than 2 million Americans visit the Dominican Republic every year. But the mysterious events have already led to a vast uptick in the number of travelers canceling or modifying their plans.