Coates said she had felt “incredibly alone” and “deserted by the British authorities” throughout her campaign for justice for her daughter, and urged the government to give her their full backing.
Seiger said Lucy Harrison’s family wanted the case lifted beyond routine consular handling.
Coates said she was not asking the Foreign Office to interfere in the American justice system, but wanted ministers to publicly support her family.
The family have requested a meeting with Foreign Secretary Ed Miliband and other senior officials.
“I think, save Sarah Hall, my MP, who has done everything in her capacity to raise and escalate Lucy’s case, it’s just felt like there’s been doors shut at every single opportunity, and people haven’t been there for us,” Coates said.
A Foreign, Commonwealth and Development Office spokeswoman said: “Our thoughts remain with Jane and her family for the tragic loss they have suffered.
“We were in contact with the family and provided consular support.
“We will always look to learn lessons to improve the consular support we offer to other families in the future.”
It is understood Foreign Office officials are reviewing recent correspondence from Lucy Harrison’s family and intend to respond.