“In the 18 years since that child’s abduction, we have received and followed up on more than 2,500 investigative leads,” Sheriff Williams said, recalling an “intense, lengthy, detailed, multiagency investigation.”

The family reached a legal settlement with the hospital two years later. And every year, the baby’s mother would wrap a slice of birthday cake in tinfoil and freeze it. The case also went cold.

Late last year, two fresh tips came in, and they led cold-case detectives to Walterboro, S.C., a town of just 5,000 people an hour west of Charleston. There, the investigators found a young woman who had been born on July 10, 1998, just like Kamiyah, but with a different name. Her documents were fraudulent, Sheriff Williams said, and “interviews with people” supported the idea that the two women were one and the same.

The detectives asked Ms. Manigo for a DNA sample. “And of course, like someone who understands their rights, she said: ‘What is this about? Do you have a warrant?’” said her lawyer, Justin Bamberg.

The investigators returned with one. Ms. Manigo gave her DNA and in short order found out the truth: She was someone else’s child.

Conscious of the fact that the woman she knew as her mother will now face trial for kidnapping, Ms. Manigo is unwilling to discuss a lot about the case, including how her family life finally unraveled. She does not want to say which name she plans to use in the future, and she insisted that she was never suspicious — although the police said otherwise at a news conference on Friday.