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The fire road inside Del Valle Regional Park's Camp Arroyo is the area where police believe a mother stabbed her 7-month-old son to death in Livermore, Calif., on Saturday, April, 26, 2014. (Doug Duran/Staff)

PLEASANTON - The 23-year-old mother accused of stabbing her infant son to death with a pocket knife admitted to the killing in a police interview, and offered an apology, according to court documents released Tuesday.

"I'm sorry, I killed my baby," Ashley Renee Newton said according to a police probable cause statement.

The San Jose woman was charged Tuesday with murder with use of a knife for allegedly stabbing to death her 7-month-old son, identified for the first time as William Brian Sherman. Newton is expected to be arraigned in a Pleasanton courtroom Wednesday afternoon.

Ashley Newton, 23, of San Jose.
Ashley Newton, 23, of San Jose. ( Courtesy East Bay Regional Parks )

First responders, including off-duty Livermore police Chief Michael Harris, arrived to find a grisly scene at Del Valle Regional Park in Livermore on Sunday morning. Harris, who was visiting the park with his young daughters, said a blood-covered Newton ran up to his car screaming "Take him, take him!" before handing him the wounded boy.

William appeared lifeless, authorities said.

According to police, Newton earlier crashed her Honda Civic near Camp Arroyo in the park, while the baby boy sat in his car seat. At some point, she took out a pocket knife and stabbed the baby boy multiple times, police said. William died at the scene.

Newton was treated for a self-inflicted wound to her arm before she was arrested.

A probable cause statement by Tracy Desiderio of the East Bay Regional Park District police said Newton "made several spontaneous statements indicating she killed the baby."

She is being held without bail at Santa Rita Jail, where she has been kept in isolation for her own protection.

Contact Malaika Fraley at 925-234-1684. Contact David DeBolt at 510-262-2728. Follow them at Twitter.com/malaikafraley and Twitter.com/daviddebolt.