PLYMOUTH, Mass. (AP) – A 21-year-old woman and 16-year-old boy who fell out of a canoe in near-freezing weather late at night were presumed dead after a search at a pond Sunday turned up no signs of them, authorities said.

Two other canoeists who were in the same boat when it overturned Saturday night were pulled out of Great South Pond just after midnight, and were recovering in hospitals.

A short time after the rescue, authorities recovered the canoe along with a sneaker and a baseball cap floating near shore, Plymouth Fire Chief Jim Pierson said.

An initial four-hour search for the missing canoeists shifted from a rescue attempt to a recovery operation Sunday morning at 8 a.m. as more than a dozen arriving divers looked for bodies in the chilly water and a helicopter circled overhead. There were no additional signs of the missing canoeists by nightfall, and the recovery effort was to resume this morning.

Relatives of the survivors told investigators the group attended a birthday party Saturday night for a 78-year-old grandmother of one of the four, then decided to head to the pond, Pierson said. The party was in Braintree, about 35 miles northwest of Plymouth.

Authorities were unsure what led the group to go canoeing late at night in temperatures in the 30s, apparently without life jackets or flotation devices, Pierson said.

“My sense is that they felt it was a way to complete the evening,” he said.

Authorities did not immediately release the names of the missing canoeists or the two male survivors. The missing 21-year-old woman was believed to have been the girlfriend of one of the survivors, Pierson said.

The green canoe apparently capsized, but investigators were unsure why, Pierson said. An inspection revealed no obvious problems with the boat. The pond stretches more than a half-mile across, and is as deep as 50 feet in some spots.

Rescue workers were called to the scene at 11:42 p.m. after neighbors reported they heard screaming. The pond is surrounded by summer cottages on the outskirts of Plymouth near Myles Standish State Forest, about 45 miles southeast of Boston.

One man who was pulled from the water was taken by medical helicopter to Massachusetts General Hospital in Boston. The other was being treated at Jordan Hospital in Plymouth.

Both were being treated for symptoms of hypothermia, Pierson said. Their conditions were not released.