The Illinois State Police took steps on Tuesday to defend the actions of a suburban Chicago police officer who killed an armed security guard on Sunday, claiming that the guard was not wearing a uniform and ignored verbal commands to drop his weapon. But witnesses have contradicted that account, and it was not clear how the State Police reached its conclusions.

The findings by the state’s Public Integrity Task Force, the lead agency in the case, were based on a preliminary investigation into the killing of the guard, Jemel Roberson, 26, who was responding to a shooting at a bar. But a lawyer for his family disputed the state’s account and criticized the agency for rushing to judgment just days after the deadly encounter.

“We are three days into this and they are saying preliminarily that it was a good shoot?” the lawyer, Greg Kulis, said in an interview on Wednesday. “They traditionally take nine months or longer.”