Jane Coates asked UK Foreign Secretary Ed Miliband to press US authorities to review the January 2025 shooting that killed her 23-year-old daughter, Lucy Harrison, in Texas.
A Cheshire coroner earlier this year ruled Lucy was unlawfully killed and said her father Kris Harrison's actions amounted to gross negligence manslaughter, findings Coates says warrant a fresh US investigation.
The push follows a Texas grand jury's decision last year not to prosecute Harrison, who admitted holding the pistol but was never arrested after authorities said they could not prove intent.
The family's case centers on what it calls an inadequate original inquiry: police did not breathalyze Harrison despite smelling alcohol, and a ranger told him the gun would fire only if the trigger was pressed.
Robert Buckland and Labour MP Sarah Hall are also urging intervention, widening pressure on the new UK government over both the US case and consular support given to Lucy's family.