One of the four people killed in a mass shooting at an Arkansas grocery was a 23-year-old nurse who was shot dead while treating another victim, police said on Sunday.

"Instead of fleeing the store, she stopped to render aid in one of the most selfless acts I've ever seen," Colonel Mike Hagar, director of the Arkansas State police, said of the nurse, Callie Weems.

The gunman opened fire with a shotgun shortly before midday on Friday at the Mad Butcher grocery in Fordyce, a town of 3,200 people about 70 miles (110 km) south of Little Rock, killing four people and wounding nine.

The suspect gunman, identified as 44-year-old Travis Eugene Posey, was also wounded in a shootout with police and will be charged with four counts of capital murder, officials said.

Weems, who comes from three generations of nurses, loved to ride horses and was nicknamed "Woodrow" after the character Woodrow Call from the book and television series "Lonesome Dove," the Arkansas Democrat-Gazette reported, citing an interview with her father, Tommy Weems.

"She died doing what she always does: Helping," Weems said.

Investigators have yet to determine a motive for what Hagar called a "completely random, senseless act."

He said the shooter began firing indiscriminately in the parking lot and then inside the store, armed with a 12-gauge shotgun and a pistol and firing mostly buckshot.

"During the incident, we observed the very best and the very worst of humanity," Hagar said, praising six responding police officers who put themselves between the shooter and civilians.

Two of the wounded were officers, police said.

Besides Weems, the fatal victims were identified as Shirley Taylor, 62, Roy Sturgis, 50, and Ellen Shrum, 81.

The surviving victims range in age from 20 to 65.