Riibe later told police he had rescued Konanki from rough waters, but lost her sight in the darkness, before they parted ways.
However, authorities said he has provided several contradictory versions of the night's happenings.
While the case was initially suspected to involve drowning, Dominican officials have not ruled out foul play and named Riibe a person of interest in the case.
He is under 24-hour surveillance as the investigation continues.