A Caribbean cruise has taken a deadly turn ... with the FBI investigating a suspected homicide aboard a massive cruise ship that was forced to return to port in Texas.

Federal authorities say they're investigating a possible murder aboard the MSC Seascape, which docked in Galveston Monday morning after departing Sunday for a weeklong Caribbean vacation.

The ship was only hours into its voyage when it turned around at around 12:20 AM Monday morning ... returning to port at the direction of authorities.

MSC Cruises confirmed the unexpected detour, saying in a statement ... "On reporting a guest incident aboard our ship, we were instructed by the authorities to return to the Port of Galveston."

The cruise line says it's cooperating fully with investigators and providing support to those affected ... while the FBI is working alongside the U.S. Coast Guard, port police and Customs and Border Protection.

Authorities have not identified the deceased person or revealed how the death occurred ... and no arrests have been publicly announced.

Despite the investigation, the ship has since left Galveston and resumed its planned itinerary.