On Friday, the Philadelphia District Attorney's Office confirmed Jacqueline Maguire would not be charged but did not say why.
SEE ALSO: Protest held after off-duty FBI special agent shoots dog in Center City Philadelphia
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Investigators say the off-duty agent was walking her dog near 16th and Spruce streets on Feb. 20 when two other leashed dogs walked by and one attacked.
Maguire then fatally shot the other dog named Mia, a 7-year-old pit bull.
The family says Mia died in her owner's arms in a police van on the way to an emergency clinic.
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The FBI has not yet said if their investigation into the incident is also closed.
The shooting led to protests in Philadelphia.
"People just assume that pit bulls are aggressive, but in reality, they are very, very loving. Just give them a chance and obviously don't shoot them dead," said Mansi Bhagwate, the founder of Revolution Philadelphia, an animal rights group.
Maguire is the special agent in charge of the FBI in Philadelphia