Pictured: 17 victims gunned down in school massacre
The 14-year-old was announced to have died by the Mormon Church, where she was a member.
The Church of Jesus Christ of Latter-day Saints said Alaina Petty was among the deceased, and that 17-year-old Madeleine Wilford, another member of the church's Coral Springs congregation, was wounded.
"Once again we find ourselves as a nation and as communities faced with a tragic loss of life and incomprehensible sorrow," the church said.
"We unite our prayers with millions of others who are mourning and praying for them."
Her aunt wrote: "There are no hashtags for a moment like this, only sadness."
She was also a member of the Junior Reserve Army Officer Training Corp, and volunteered in the aftermath of Hurricane Irma.
Her family said: "Alaina was a vibrant and determined young woman, loved by all who knew her. Alaina loved to serve.
"Alaina was part of hundreds of volunteers that rushed to the most heavily impacted areas of Florida to clean up and help rebuild the lives of those devastated by Hurricane Irma. Her selfless service brought peace and joy to those that had lost everything during the storm.
"While we will not have the opportunity to watch her grow up and become the amazing woman we know she would become, we are keeping an eternal perspective."