A marine unit of the Royal Turks and Caicos Islands Police Force had intercepted the packed sloop and was escorting it to shore when it abruptly overturned, sparking a frantic search and rescue operation in the pre-dawn darkness.
Karlo Pelissier, the Haitian consul to the Turks and Caicos, said he was told by survivors several migrants attempted to jump off the 28ft boat and flee to land as they neared the island of Providenciales and the surge caused the overloaded sloop to overturn.
The authorities rescued 33 migrants, including one 12-year-old boy, and recovered the bodies of 17 people.
Divers and US Coast Guard helicopters assisted in a search for additional survivors or victims in the area where the incident occurred, about 150 yards from shore.
The remains of the 12 men and five women killed are to be repatriated to Haiti in the next few days.