A second reporter, Jesse Walden, and the girl's mother were shot and injured by the same gunman, who is also suspected of the other homicide.
The journalists were covering the killing of a woman when the teenage suspect returned, police say.
It is unclear if they were targeted.
The suspect was armed when he was arrested and was not co-operating with police, said investigators.
The other two victims in Wednesday's two attacks in Pine Hills, a suburb west of Orlando, have not yet been identified.