PHOENIX -- Phoenix police arrested a 21-year-old Susan Pierce on Wednesday night and charged her with child abuse and obstructing an investigation in the beating death of her 20-day-old daughter.

The child’s father, 21-year-old Sean Dexter Pierce, was arrested last week and charged with the child’s death.

Susan told police last Monday that she had left her newborn, Angel, in her husband's care and went to take a nap.

"She was doing fine, then she wasn't breathing," she said.

Court documents indicate that when police arrived, they found the infant had suffered multiple bruises and lacerations.

Angel was rushed to St. Joseph's Hospital, where doctors found massive internal injuries and a partial skull fracture.

The baby was declared brain dead and died one day later.

Susan was asked if her husband was capable of doing anything like this. "I wouldn't think he would abuse her," she told CBS 5 News.

According to court documents, Sean Dexter Pierce admitted to police that he shook the baby.

He faces charges of second-degree murder and child abuse, and is being held in the psyche ward on a $1 million bond.

Susan was charged with one count of child abuse for failure to protect the baby and one count of obstructing an investigation.