A U.S. woman has confessed to stealing a baby 23 years ago and raising the girl as her own daughter, until the abducted woman discovered the truth on the Internet, according to court documents.

Ann Pettway said she kidnapped Carlina White in August 1987 from Harlem Hospital in New York City.

Pettway was 19-years old at the time. She had suffered several miscarriages before the abduction and desperately wanted a child, according to a written statement she gave to the U.S. Federal Bureau of Investigation.

FBI agent Maria Johnson gives the contents of the statement in court documents filed Monday.

"Pettway took the victim from the victim's family and this was totally unacceptable," Johnson said. "Pettway is truly sorry."

Lawyer Robert Baum, who is representing Pettway, said she faces a mandatory minimum sentence of 20 years behind bars – or up to a life sentence -- if convicted.

"She feels badly. She's very upset," he told reporters. "She's expressed concern about her family. But she understands the gravity of the charges."

White was 19-days old when she was snatched from the hospital. Her parents, Joy White and Carl Tyson, had wanted their baby treated for a high fever. They left the hospital to rest, and White had disappeared when they returned the next day.

Police never found White. According to Johnson, she was raised by Pettway in Connecticut under the name Nejdra Nance.

As a teenager, White became suspicious of her mother when Pettway couldn't give her a birth certificate or social security number, and she wondered why she looked so different from other family members.

This month, at the age of 23, White's suspicions were confirmed when she searched through pictures of missing persons on the U.S. National Center for Missing and Exploited Children website.

She came across an image that looked similar to her, and the centre gave her contact information for Joy White. DNA tests confirmed that the two were, in fact, mother and daughter.

"This is a one-in-a-million kind of thing," said Ernie Allen, the centre's president. "This is the longest stranger abduction case we're aware of that's ever been resolved."

White's father, Carl Tyson, said he was shocked and happy to be reunited with his daughter after more than two decades.

"I have my whole puzzle. I have all my four kids now," he told NBC's Today show.

But he added, "I know I can't do all the things I wanted to do when she was little."