LIVERMORE, Calif. - A mother believed to be suffering from depression admitted to investigators that she stabbed her 7-month-old son to death in a Northern California park, police said Sunday.

Ashley Newton, 23, of San Jose, was arrested Saturday on suspicion of murder, the East Bay Regional Parks District Police Department said in a statement.

"This is an extremely shocking case for us," said Chief Timothy Anderson of the park's police department.

The motive remains unclear, but Anderson said Newton had a history of depression and appeared to have self-inflicted knife wounds on her wrist. Investigators continue to interview her family and friends.

Police were called at 10:30 a.m. Saturday on reports of a damaged Honda sedan in the Del Valle Regional Park east of San Francisco. They found the car abandoned with the engine running and an empty child's seat.

Officers were about to tow the car two hours later, believing it was involved in a hit-and-run, when Newton walked up to them holding her lifeless baby, Anderson said.

Officers tried to resuscitate the baby, but he was pronounced dead at the scene.

Newton was arrested after she made statements implicating herself in the stabbing, Anderson said, adding that investigators found a knife in the area.

Newton remains in jail with no possibility of release on bond, police said, and is due in court Tuesday.