Court documents show a $1,000 donation was made in Dan Webb's name to Foxx's campaign in 2016.
Webb's law firm hosted a fundraiser for Foxx, but he says he does not recall making the donation.
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Foxx said she just learned about the donation last week and continues to cooperate with the investigation.
Webb is now investigating Foxx's decision to dismiss felony charges against Smollett, who's accused of staging an attack on himself.
WATCH: Video shows Jussie Smollett with rope around his neck
Smollett was charged with staging the attack in Chicago's Streeterville neighborhood in January. Police and prosecutors said Smollett orchestrated the attack with the help of two brothers. One brother was an extra on "Empire" and the other was Smollett's personal trainer.
All charges against Smollett were dropped in late February in exchange for community service and forfeiture of his $10,000 bond payment.
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