TV reporter Dylan Lyons, 24, and a nine-year-old girl have been fatally shot near Orlando, Florida, close to the scene of a murder that took place hours earlier.
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.
In a news conference, Orange County Sheriff John Mina said the journalists were "in or near their vehicle", which he said did not look like a TV station's official vehicle, when they were attacked at around 16:00 local time (22:00 GMT).
He said the Spectrum News 13 journalists had been reporting on a shooting that took place earlier in the day at around 11:00 local time, which saw a woman in her 20s fatally shot inside a car, when the suspect returned to the crime scene and opened fire.
After attacking the journalists, the alleged gunman - Keith Moses, 19 - went into a nearby home and shot the girl and her mother, the sheriff said.
The mother was in hospital in a critical condition, he added.
Other journalists nearby helped provide first aid to the victims, according to local reporters.