A suburban Chicago police officer responding to a shooting Sunday at a bar killed a security guard who was just "doing his job," an attorney for the slain man's family said. An officer from Midlothian, Illinois, fatally shot Jemel Roberson, a 26-year-old security guard working at the bar, Cook County Sheriff's Office spokeswoman Sophia Ansari told CNN. Law enforcement agencies responded to the shooting around 4 a.m. at Manny's Blue Room Lounge in Robbins, about 20 miles south of Chicago, Ansari said. Three people and an alleged shooter were injured, she added. Then, after Roberson had apprehended an alleged gunman, the Midlothian officer shot Roberson outside the bar, according to a federal lawsuit filed Monday that calls the fatal shooting "excessive and unreasonable" and says it violated Roberson's civil rights. Witnesses said security had asked several drunk men to leave the bar, and at least one person returned and opened fire, CNN affiliate WGN reported. A bartender was among those victims, according to the lawsuit.