The New Hampshire State Police are investigating the death of a Concord, NH woman who died in the Merrimack River on Sunday.

Two bystanders attempted lifesaving measures until first responders arrived, but efforts to revive her were unsuccessful.

A Concord woman died Sunday after suffering a medical emergency while swimming in the Merrimack River, authorities said. Claire Knowles, 67, of Concord was floating in a tube in a mild current in the river and was returning to shore when she suffered a “medical event” around 11:52 a.