The Long Island resident made the discovery on Thursday, Nov. 3, according to Suffolk County Police.
The girl traded candy with her classmates at PJ Gelinas Junior High School in East Setauket over the past few days before finding the blade in a mini 3Musketeers bar, according to police.
The girl's mother immediately reported the incident to police just after 7:30 p.m. Thursday.
The girl went out trick or treating on Halloween on Monday, Oct. 31 in the Stony Brook area.
Police are asking residents to make sure adults are present when children open candy they received from trick or treating, especially in Stony Brook and Setauket.
Officers are asking anyone with information to call the Sixth Precinct Crime Section at 631-854-8626, or anonymously to Crime Stoppers at 1-800-220-TIPS.
Check back to Daily Voice for updates.
Click here to follow Daily Voice Windham-Willimantic and receive free news updates.