A newborn baby boy snatched from an idling car in a South Carolina post office parking lot was found safe early Wednesday and a woman was in custody in his kidnapping, police said.

Investigators tracked down the 1-month-old infant, Angel Miguel Perez, at a home in Summerville at about 12:30 a.m., said North Charleston police spokesman Spencer Pryor.

A 19-year-old woman was arrested and charged with kidnapping, Pryor said. Police identified her at a Wednesday news conference as Andrea Simone Walker and said she began following the baby's mother, 26-year-old Lidia Juarez, at some point on Monday.

Juarez said she stopped at a North Charleston post office on Monday afternoon and left the infant in the car with its engine running while she went inside to buy stamps.

Juarez told police she was in the post office less than five minutes. When she came back to her car, her son was gone.

A woman was spotted running across the parking lot with a baby, according to media reports.

Police said Walker was able to get into Juarez's vehicle while she was in the post office and remove baby Angel from his car seat. Walker then fled the area.

State officials issued an Amber Alert for the child late Monday and local, state and federal authorities asked for public help in the search Tuesday.

North Charleston and Summerville officers were acting on a tip when they went to the house where the child was found, Pryor said.

The boy was taken to the Medical University of South Carolina hospital in Charleston for an evaluation and then was reunited with his parents.

The FBI were offering a $20,000 reward for information that leads to the conviction of the kidnapper and set up an e-mail address and a bank of five phone numbers to encourage anyone with information to contact them.

It wasn't immediately clear who the tipster was or whether he or she will be able to collect the reward money.

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The Associated Press contributed to this report.