Jake Patterson, 21, is being held pending charges of first-degree murder and kidnapping.
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After citing him for marijuana use, an officer escorting the suspect off the train felt a weapon inside his waistband, police said.
McHenry County sheriff's officers found about 1,750 grams of marijuana, two semi-automatic handguns with magazines and ammunition and $5,237 in cash.
The two had been arguing when Abdullah pulled the gun out, pointed it at the male friend and loaded a round, police said.
The criminally charged 14th Ward aldermen isn’t running for mayor, but his presence loomed as large at the Northwest Side forum.
A man being held at the Cook County Jail in an unrelated case has been charged with murder in the July 11, 2018 shooting of Lorenzo Williams.