Officers rushed to the family's home on Westwood Drive when they got the call Sunday morning, and say they found a lifeless 2-year-old girl in a bathtub filled with water.
The girl was rushed to Saint Louise Regional Hospital, where she was pronounced deceased.
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The child's mother, 35-year-old Marcie Montelongo was arrested.
She's being held in the Santa Clara County Jail for murder.
In addition to the Gilroy Police Department, the Santa Clara County District Attorney's Office and the Office of the Medical Examiner-Coroner are involved with the investigation, as per the Santa Clara County Child Death Protocol.
"This investigation is active and ongoing," police said.
The victim's identity will not be released until family members are contacted.
Police ask that if you have any information about this case, to please contact Gilroy Police Department Detective Chris Silva at (408) 846-0335.