Dominican Republic
Nathaniel Holmes (L), Cynthia Day (NBC Washington)

The bodies of a Maryland couple who died under mysterious circumstances in the Dominican Republic have been returned to their home state of Maryland, where the families are arranging to have their own private autopsy performed.

The bodies of Edward Nathaniel Holmes, 63, and Cynthia Ann Day, 49, have been returned to the Washington-area suburb of Temple Hills, Md., family lawyer Steven Bullock told PEOPLE.

READ MORE: Second Black American couple found dead in Dominican Republic in two months

On top of arranging for private autopsies, the families also plan to review toxicology reports, Bullock said.

“The families are determined to find out what happened and why,” Bullock told the news organization. “At this time, the cause of death remains a mystery.”

Earlier, the State Department confirmed that the FBI would conduct an investigation into the deaths of the couple, as well as into the string of deaths of American tourists in the Caribbean country on the island of Hispaniola.

The bodies of Holmes and Day were found in their hotel room at the Grand Bahia Principe La Romana resort on May 30th, five days after they’d arrived. The hotel staff discovered the bodies after going to check on the couple after they missed their scheduled check out, PEOPLE reports.

READ MORE: New York couple who went missing in Dominican Republic confirmed dead

The couple suffered from respiratory failure and pulmonary edema, the Dominican Republic National Police have said, based on earlier autopsies conducted by the Dominican National Institute of Forensic Sciences. Day also suffered a cerebral edema.

Medication to control high blood pressure was found in the couple’s room, WBAL has reported.

The FBI was brought in by the U.S. Embassy in Santo Domingo, the capital of the Dominican Republic, and the country’s Ministry of Tourism to help investigate, USA Today reported.

Police in the Dominican Republic are working not only with the FBI but also with the U.S. Centers for Disease Control and Prevention in Atlanta, according to the news organization.