SAN FRANCISCO (AP) — The mother of a 13-year-old California girl who was declared brain dead after suffering complications from sleep apnea surgery said she has seen changes in her daughter's condition that give her hope.

Doctors have declared Jahi McMath brain dead, and medical experts say she cannot be revived.

However, the girl's mother, Nailah Winkfield, said in a letter posted Wednesday on Facebook and Twitter that she believes her daughter is doing better since she was removed from Children's Hospital Oakland last month and taken to an undisclosed facility.

"Despite what people say about my daughter being dead and how I must be ignorant not to get that, I can tell you that she is much better physically since she has left Children's Hospital and I see changes that give me hope," Winkfield said in the letter.

She didn't give details of Jahi's condition or disclose her location. Jahi's family has said the girl has been fitted with feeding and breathing tubes while remaining on a ventilator.

Jahi underwent tonsil surgery on Dec. 9 and began bleeding heavily before going into cardiac arrest. Doctors declared her brain dead on Dec. 12 based on exams and tests showing no blood flow or electrical activity in her cerebrum or the brain stem that controls breathing.

Winkfield has refused to believe Jahi is dead and went to court to prevent the hospital from taking her daughter off a ventilator.

Medical experts have said a ventilator won't work indefinitely for Jahi and caring for a patient whom three doctors have said is legally dead is likely to be challenging.

The bodies of brain-dead patients kept on ventilators gradually deteriorate, eventually causing blood pressure to plummet and the heart to stop, said Dr. Paul Vespa, director of neurocritical care at the University of California, Los Angeles, who has no role in Jahi's care. The process usually takes only days but can sometimes continue for months, medical experts say.

In her letter, Winkfield also expressed gratitude for the emotional and financial support she has received from people around the world.

"If I had it my way, I would say thank you to each and every person in their native language so they could understand how much I appreciate them for all their support and, most importantly, prayers," Winkfield wrote.

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(Photo:  REUTERS/Beck Diefenbach)