Michael McLendon, who lived with his mother, killed five of his victims in Tuesday's shooting spree.
Those victims included the wife of a local deputy sheriff and her child, at a mobile home in Samson, in southeastern Alabama, according to a local coroner and Wynnton Melton, mayor of nearby Geneva.
His other victims included two people killed at a convenience store and a man in a pickup truck who died during a car chase as McLendon apparently fired at random, said Melton.
"We sent 11 bodies to Montgomery (the state capital)," said Robert Preachers, the coroner in neighboring Coffee County who said he knew most of the victims including the killer.
"He (McLendon) was a nice quiet kid, no trouble. He was always polite and nice," Preachers said. Asked about a possible motive, he said: "He carried it with him to the grave."
Rural southeastern Alabama, which borders Florida, is a largely agricultural area with many low-income families.
Mass shootings have become more frequent in the United States in recent years.
Guns are widely available for purchase in the United States, a country that prides itself on the right to own weapons for self defense and hunting.
Mother Killed With Family Dogs
The website of the Dothan Eagle newspaper quoted Coffee County District Attorney Gary McAliley as saying the gunman had been armed with both pistols and assault rifles.
The shooting spree began in Kinston, a small town in Coffee County, and continued to Samson, where McLendon killed his grandmother, uncle and others, said the FBI, local police and Preachers.
It ended after a car chase and gun battle in Geneva, the county seat about 12 miles (20 km) away, they said.
Describing the car chase, Melton said: "Officer Ricky Morgan rammed his car to distract him and was rewarded with a hail of bullets .... One bullet grazed the shoulder of police chief Frankie Lindsey."
A police officer in Geneva said the gunman "shot at several vehicles on the highway and then he shot at Wal-Mart and Piggly Wiggly," a grocery store.
McLendon also shot the family's dogs, Preachers said. "He killed her (his mother) and her dogs. He shot the dogs in the head. Then he piled them up and gassed them (poured gasoline on them and lit it). He laid the dogs next to her," he said.
In one of the worst recent shooting incidents, a gunman dressed as Santa Claus killed nine guests at a Christmas Eve party, before taking his own life in Covina, California, a suburb of Los Angeles.
On April 16, 2007, Virginia Tech, a university in Blacksburg, Virginia, became the site of the deadliest rampage in modern U.S. history when a student gunman killed 32 people and himself.
In Germany on Wednesday, a 17-year-old gunman went on a shooting spree at his former school in the southwest of the country, killing up to 15 people before dying in a shootout with police, authorities said.
