Their bodies were found by a hotel worker later that day, with no signs of violence, news reports said. Several bottles of heart medication were found in their room, according to the reports.
The State Department is monitoring the investigation and has been in regular contact with Dominican authorities, a department official said Friday.
“We stand ready to provide all appropriate consular assistance,” the official said.
A Diaro Libre news website report on Friday said Holmes called for a hotel doctor Thursday morning, complaining that he felt ill. When the doctor appeared, he refused treatment, the news agency said, citing the Dominican Republic’s National Police.
On Saturday, a spokesman for the police said autopsy results for Day and Holmes were pending and that the case is still under investigation.
This story has been updated.
Aaron C. Davis contributed to this report.