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A New Jersey bus monitor has been charged with manslaughter after a six-year-old girl in her care died when she was strangled by a safety harness.

Prosecutors say Amanda Davila, 27, had headphones on and did not notice Faja Williams could not breathe.

Faja had a rare disorder, Emanuel syndrome, which meant she could not speak or walk, but could make sounds.

Ms Davila, who appeared in court on Thursday, also faces child endangerment charges.

Officers responding last Monday to a 911 call from Claremont Elementary School in Franklin Park found the girl unresponsive.

The child was taken to a nearby hospital, where she was pronounced dead.

Investigators determined that Faja began to slump in her wheelchair as the bus hit bumps in the road.

A harness used to secure the girl became "tight around her neck, ultimately blocking her airway", said Somerset County Prosecutor's Office.

"Amanda Davila was seated toward the front of the bus and was utilising a cellular telephone while wearing ear bud headphones devices in both ears," the statement added.

"The investigation revealed that this was in violation of policies and procedures."

Ms Davila was arrested on Wednesday.

In an interview with CBS, the ����ý's US partner, Faja's parents said their grief is unimaginable.

"This lady [Ms Davila] is on the cell phone. [Faja's] back there fighting for her life," her father, Wali Williams, said.

"She's not even looking back."

Faja's mother, Najmah Nash, said she got the call from authorities about 45 minutes after her daughter was picked up from home.

"My sweet baby, she was six years old, and she was the sweetest kid you'll ever meet," said Ms Nash.

The firm that operated the bus, Montauk Transit LLC, told the ����ý they were "devastated" by Faja's loss.

The company will "support any punishment that the justice system determines appropriate for the bus monitor who has been arrested", said its president, John Mensch.