The Montgomery County District Attorney's Office said 50-year-old Ruth DiRienzo-Whitehead is charged with first-degree murder in the death of Matthew Whitehead.
Police were called to the family's home just after 7 a.m. Tuesday after the boy's father found him dead.
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According to the D.A.'s office, an autopsy confirmed Whitehead had been strangled.
Investigators said DiRienzo-Whitehead killed her son after he went to sleep around 9:30 p.m. Monday, then drove the family's SUV to Cape May, New Jersey.
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She drove the vehicle into the ocean, investigators said, and once the vehicle could no longer be driven, she walked to Wildwood Crest, New Jersey.
That's where DiRienzo-Whitehead was taken into custody. She was then questioned by Montgomery County detectives and Horsham, Pennsylvania police.
A search of the SUV turned up a black men's dress belt on the driver's side floor of the vehicle, the D.A.'s office said.
There was no immediate word on a motive for this killing.
DiRienzo-Whitehead is currently being held in Cape May County and is awaiting extradition to Montgomery County.