Cook County prosecutor Erin Antonietti says Sengstacke approached security officers near Obama's South Side home Tuesday and told them he needed to speak with the senator about a job.
Sengstacke was asked to leave. He went back to his BMW, and then approached the officers again.
Antonietti says when police searched his car, they found a .40 caliber pistol, a bulletproof vest and ammunition. Sengstacke was charged with unlawful use of a weapon by a felon and criminal trespassing on state land.
Authorities say Sengstacke did not make any threats against Obama.
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