NEWBURYPORT, Massachusetts — A family fishing trip to a Massachusetts river ended in tragedy with a mother dead and her 6-year-old son missing, authorities said.

The family of six — a mother and father and their four children — were fishing from Deer Island in the Merrimack River when just after 7 p.m. Thursday, two of the children entered the water, Newburyport fire and police officials said.

The mother entered the water to get the children, and with the help of a good Samaritan captain of a fishing vessel who saw that the family was in distress, a 7-year-old girl was rescued, authorities said.

But the mother was unresponsive, and after life-saving efforts were performed at the scene, she was taken to Anna Jaques Hospital in Newburyport, where she died, authorities said.

Rescuers could not find the 6-year-old boy. All other members of the family are accounted for.

Local, state and federal agencies searched through the night, and about 10 divers with the Massachusetts State Police dive team reentered the water on Friday morning with specialized equipment to continue the search for the boy.

No names were released.