Grenadian police Commissioner Don McKenzie said the prisoners had escaped custody on Feb. 18 and the following day commandeered a catamaran named Simplicity - with U.S. citizens Kathy Brandel and Ralph Hendry on board.

"Information suggests that while traveling between Grenada and St. Vincent, they disposed of the occupants," McKenzie said. The couple was last seen on the night of Feb. 18 and their boat was later tracked leaving the island late at night at an unusual speed.

The three fugitives were captured by police in neighboring St. Vincent and the Grenadines on Wednesday, he said, adding that a team from Grenada had been dispatched to collaborate on "having a complete and thorough investigation of the matters at hand."

The countries are separated by about 105 km (85 nautical miles).

Ron Mitchell, 30; Trevon Robertson, 19; and Abita Stanislaus, 25, were being held at the South Saint George Police Station near the island's southwestern tip on charges of robbery with violence, when they escaped, police said last week.

Mitchell, they said, also faced counts of rape, attempted rape and indecent assault.

Police from St. Vincent said in a statement they had discovered the ship with no bodies but items strewn across the deck and possible blood on board.

On Monday, they said, the suspects appeared in court on four immigration counts, to which they pleaded guilty, and they had been remanded into custody with sentencing set for March 4.

St. Vincent police Superintendent Junior Simmons said that though the couple are presumed dead, "the investigation and search for the missing persons continues."

(Reporting by Robertson S. Henry in Kingstown and Sarah Morland in Mexico City; Editing by Leslie Adler)