Last year alone, there were 2,300 nonfatal shootings in Chicago. The police made arrests in just 141 of them — a “clearance” rate of about 6 percent. And though Mayor Brandon Johnson pledged during his campaign to hire 200 more detectives, records show the number of detectives assigned to at least one shooting actually has fallen by nearly 20 percent. Experts say the chronic lack of arrests is a big part of the reason for as many shootings as there are in many Chicago neighborhoods plagued by gunfire.