Little Elm Police Chief Rodney Harrison says Detective Jerry Walker died Tuesday night at a Denton hospital hours after the shooting.
Meanwhile, a standoff continued at a house with an armed man suspected of firing the fatal shot.
Authorities say the episode began about 3 p.m. Tuesday with the report of a man outside a house, armed with a long gun. Denton County sheriff's Lt. Orlando Hinojosa says officers arrived to find an armed man screaming at them from a back yard.
As officers withdrew, the man ducked into the house and fired from a window, striking Walker.
Tactical squad officers arrived and a standoff ensued.
The scene is still active.