The woman had a bump on her head, but no other signs of foul play were found, said BART spokeswoman Cheryl Stalter.
The woman's body was discovered about 5:30 a.m. near the passenger pick-up/drop-off area at the station.
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When police arrived, emergency crews were attempting to treat the woman, but she was declared dead at the scene.
Santa Rita Jail officials were reviewing their records to determine whether the woman had a bump on her head when she was released. In addition, BART police are pulling images from surveillance video that may be relevant to the incident, Stalter said.
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