A 12-year-old boy died when a lesson in sailing safety took a tragic turn Tuesday.

Suffolk County police said the Northport boy was in the middle of a group sailing lesson at the Centerport Yacht Club, at 33 Beach Plum Drive, when he was struck by a boat propeller.

The town harbor master received a call around 3 p.m. about a child injured underneath a boat, a town spokesman said.

Three children, all wearing life vests, were taking part in a sailing lesson when their sailboat was capsized as a controlled part of the lesson, police said. Capsizing safety is an essential and standard component of sailing education for both adults and children.

As part of the lesson, the fatally injured boy had been in the water. The 18-year-old instructor, who is also from the Northport area, plucked him from the water and onto a small inflatable Zodiac boat, said Det. Sgt. James Scimone at a news conference outside the Centerport Yacht Club.

The boy was sitting on the edge of the boat and fell when the boat accelerated, becoming caught in the Zodiac’s propeller.

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“It was a matter of seconds,” Scimone said.

The boy suffered “massive injuries to the chest” and did not appear to be breathing. The instructor jumped into the water and with the aid of a second person, pulled the boy into another boat and began to administer CPR, Scimone said.

EMTs from the Centerport Fire Department assisted onshore and brought the boy to Huntington Hospital, a department spokesman said. The boy was pronounced dead there, police said.

An instructor was also admitted to the hospital and treated for shock.

The other two children, who were interviewed by police, were upset, but “doing pretty well,” Scimone said. Neither child received medical aid.

Police are continuing to investigate, and Scimone said they are treating the death as an accident. The child’s identity was not released Tuesday.

“It’s a horrific accident. . . . Nobody is at fault,” Scimone said. “The instructor was doing everything he was supposed to.”

The Centerport Yacht Club did not return requests for comment.

With Lexie Schapitl

CORRECTION: An earlier version of this story incorrectly stated the age of the victim.