The teenager, from Birchwood in Warrington, Cheshire, was found by members of the public as she lay wounded on a path in Linear Park, Culcheth, at around 3.13pm on Saturday.
Two 15-year-olds, from the local area, remain in custody after being arrested on suspicion of murder, Cheshire Police said.
Brianna was a transgender girl but police said there is no evidence to suggest her murder was a hate crime.
Donations on a GoFundMe crowdfunding page set up for Brianna's family, which said the schoolgirl was looking forward to taking her exams this year, have passed £18,000.
Paying tribute to the teenager, who had a large following on social media site TikTok, her family said in a statement: "Brianna was a much-loved daughter, granddaughter, and baby sister.
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