A farmer's wife died and her son was badly injured after a raider tried to steal fuel at a County Durham farm.
Police said William and Rosemary Dove saw a man taking red diesel from a pump at East House Farm in Bishop Middleham, near Sedgefield, on Sunday night.
Mr Dove and son James gave chase to a nearby field, where the younger man was hit by the getaway vehicle.
Mrs Dove, 68, who had returned to the farmhouse to dial 999, was later found collapsed, and subsequently died.
James Dove was taken to the University Hospital of North Tees in Stockton, where his injuries are described as non-life threatening.
Mr and Mrs Dove had spotted the thief at about 2030 BST after returning from an agricultural show in Scotland.
Rosemary Dove ran to dial 999, while the two men jumped into another vehicle and gave chase.
They were joined by relatives from a nearby farm, who also drove after the raider, and the group cornered the silver Mitsubishi pick-up style truck in a field.
The couple had returned to their home from an agricultural show in Scotland
James Dove, aged in his mid-40s, got out of his vehicle to approach but was knocked into a ditch as the pick-up broke through the cordon.
Ch Insp Paul Harker said the truck, which "crashed out of the field", had not been traced by police.
He said: "At this stage it is not clear exactly how many people were in the pick-up.
"We urgently need any help that will lead us to those involved in this tragic series of events that has left the Dove family absolutely devastated."
Mrs Dove is believed to have been alone when she collapsed, and was pronounced dead after paramedics were called to the scene.
The raider is described as white, aged between 20 and 30, of muscular build with short dark-brown hair. He was wearing a checked shirt.
Police also want to trace the pick-up truck, which had a distinctive amber lighting bar on the roof.
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