The freelance journalist whose lawsuit forced the Chicago Police Department to release video of a fatal police shooting of a black teenager says he was barred from attending a news conference with the mayor and police superintendent on Tuesday.

Dozens of journalists were gathered inside Chicago’s Police Headquarters on Tuesday, listening to Mayor Rahm Emanuel and Police Superintendent Garry F. McCarthy discuss the shooting of the teenager, Laquan McDonald, 17. Absent was Brandon Smith, the freelance journalist whose lawsuit over the summer had pressed for the release of the police dashboard camera video.

In a telephone interview, Mr. Smith said that he had rushed to the news conference after hearing about it from friends, but that police officers guarding the door had blocked him from entering. He said they had told him the room was too full to allow members of the news media without credentials. Mr. Smith does not have them, he said, because he does not usually attend credentialed events.

Mr. Smith said he did not know if officials had deliberately kept him out or if it had been an oversight. But he said he should have been invited because of his role in the video’s release.