Former U.S. swimming star Jamie Cail died suddenly at 42 years old in the Virgin Islands last week ... and now, authorities say they've launched an investigation into the mysterious circumstances surrounding her death.
According to the United States Virgin Islands Police Department -- Cail, who won a gold medal at the 1997 Pan Pacific Championships, was found unresponsive in St. John by her boyfriend on Feb. 21 just after midnight.
Police say the swimmer's significant other had been out at a local bar but left to check in on her at their home -- and discovered her on the floor.
Cops say the man and one of his friends were able to transport Cail to a local hospital -- where she was administered CPR.
Police, though, say the lifesaving efforts were unsuccessful, and she was pronounced dead at 2:39 AM.
The Criminal Investigation Bureau is now investigating the death -- and officials are urging anyone with information on the case to call authorities.
Cail competed on the national stage throughout the late 1990s ... eventually joining the USC swimming team.
RIP.