An initial report by Illinois State Police contradicted some witness accounts.

The report, which Mr. Kulis has criticized, says Roberson was given “multiple verbal commands” to drop his weapon and get on the ground before he was shot by Officer Covey. It also says Mr. Roberson was wearing “plain black clothing with no markings” that would identify him as a security guard, according to witnesses. Mr. Kulis told NPR that Mr. Roberson was wearing a hat with the word “security” on it.

Midlothian and Illinois state police, who are both investigating the shooting, had declined to name Officer Covey, even as Mr. Kulis subpoenaed records of the shooting and Mr. Roberson’s family pressed for the officer to be identified.

Nick Valadez, a lawyer for Midlothian, told The Chicago Tribune that growing pressure led to the decision to release the officer’s name.

“Given that the vitriol has died down slightly and given that the name was being batted around, speculated to, as well as the plaintiff pursuing it and amending the complaint, it just seemed to be time,” Mr. Valadez said.

Neither Midlothian nor Illinois state police responded to requests for comment on Saturday. Neither Officer Covey nor a union representing him could be reached for comment.

“I want justice for my son. I want this officer put away. I want him to lose his job,” Ms. Roberson said as some supporters held T-shirts with the words “Security Guard. Don’t Shoot” and a picture of Mr. Roberson’s face.

Mr. Roberson had an infant son, and his girlfriend was pregnant with their second child. Ms. Roberson said he was a musician who loved playing the organ at church. She did not like that he worked as a security guard or that he wanted to become a police officer.