A shooter opened fire at an outlet mall north of Dallas, Texas on Saturday, and emergency officials confirmed fatalities and said at least nine wounded people were rushed to area hospitals.
Some of the victims were as young as five years old, CNN reported.
A police officer was in the mall on an unrelated call when gunfire broke out around 3.30pm (2030 GMT), said chief Brian Harvey of the Allen, Texas, police department.
"He heard gunshots, went to the gunshots, engaged the suspect and neutralised the suspect," Harvey said. "He also then called for ambulances."
Harvey confirmed there had been fatalities but declined to give a death toll, saying "we do not have an accurate count."
Allen fire chief Jonathan Boyd said his department "transported nine victims to area trauma facilities."
Texas Governor Greg Abbott called the mass shooting an "unspeakable tragedy."
Authorities initially thought there might be a second shooter at Allen Premium Outlets, a sprawling shopping complex in the city of Allen, 40km north of Dallas, CNN said.
Police combed through stores in the mall, and photos and drone video from the scene showed shoppers and store employees rushing into parking lots.
Harvey later said police believe the unidentified shooter, who CNN said was wearing tactical gear, "acted alone."
A father who arrived at the mall after his daughter, who was inside, phoned to tell him about the shooting, also told CNN that police had informed him there may have been a second shooter.
"We saw the police outside the door, and they told us we had to go, and that they are still looking for the person," Jaynal Pervez said, according to CNN.
"There's no more safe places. I don't know what to do."
With more firearms than inhabitants, the US has the highest rate of gun deaths of any developed country – 49,000 in 2021, up from 45,000 the year before.
There have been more than 195 mass shootings – defined as four or more people wounded or killed – so far this year in the US, according to the Gun Violence Archive.