Amanda Mileto, 21, injected the drug in the woman's restroom shortly before she was taken into custody on an active warrant for violating probation, police said in a news release.
Courtroom security searched Mileto and found her carrying a used needle and four empty heroin folds, they said.
They said they also found fresh blood droppings in the restroom.
Mileto was charged with possession of drug paraphernalia and a syringe, as well with being under the influence of heroin, before she was transported to the Passaic County Jail on the probation violation.
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