The investigation comes after District 3 incumbents sent campaign postcards that included the city seal, which could violate city code.
The election will be held Nov. 9 and eligible residents must register to vote by Oct. 10 through the Prince George’s County Board of Elections.
Brennan has served on the council since 2014.
District 1's incumbent candidates ran uncontested.
The Diamondback profiled everyone running and outlined voting history for those with council experience.