The missing pilot and all five passengers are U.S. citizens, Castrodad said.
Witnesses reported seeing an explosion on the plane before it crashed around nightfall Sunday about a half-mile (a kilometer) offshore, police spokeswoman Jessica Cardona said.
The Cessna 206, chartered by Tropical Aviation Corp., was traveling from the Casa de Campo International Airport in the Dominican Republic to San Juan, according to a Coast Guard news release.
Castrodad said rescue crews were hoping the search for survivors would become easier after daybreak.
CLICK HERE to follow Action News on Twitter
CLICK HERE to get Action News on your website
CLICK HERE to find Action News on Facebook