At least four other people, including a child, were injured in the shootings which took place across two Alabama counties.
The man burned down his mother's home, killed his grandparents, an uncle and aunt, and shot at apparent strangers on their porches as he drove by, authorities said. He then turned the gun on himself at a metals plant.
Frankie Lindsay, chief of police in Geneva County where five people were shot dead, said the man also fired at officers as they chased his vehicle in patrol cars.
He told BBC Radio 5Live: "We had set up a road block here in Geneva anticipating him coming down Highway 52 ... my vehicle rammed into the suspect's vehicle and slowed him down enough and he pulled out what looked to me like an AR-15 or M-16 automatic weapon and fired a burst of rounds into my vehicle and my other officer's vehicle. Some of the shrapnel from the bullets entered my shoulder."
He said the wife and child of a deputy sheriff in Geneva were among the dead.
"This is probably the worst incident in the state of Alabama that I know of. I've been in this business for 33 years and this is the worst I've ever seen."
The afternoon of bloodshed began in Kinston, near the Alabama-Florida border, where the man burned down his mother's house.
Officials found the woman's body inside the house, but were unable to get inside the still-burning house to determine if he shot her first.
The gunman then headed east, into Geneva County, where he shot and killed four adults and a child at a home in the nearby town of Samson.
Then he killed one person each in two other homes.
The identities of all the victims were unknown, but the Coffee County coroner Robert Preachers said they included other members of the gunman's family.
"He started in his mother's house," Mr Preachers said. "Then he went to Samson and he killed his granny and granddaddy and aunt and uncle."
"We don't know what triggered it," Mr Preachers added.
He then killed someone at a Samson supply store, and another person at a service station.
Samson contractor Greg McCullough said at the petrol pump when the gunman opened fire, killing a woman coming out of the building and wounding Mr McCullough in the shoulder and arm with bullet fragments that struck his truck and the pump.
"I first thought it was somebody playing," he said. He said the gunman roared into the parking lot and slammed on his brakes. Then he saw the rifle.
He said the gunman fired and the rifle appeared to jam, then he "went back to firing". Then he drove off.
Mr McCullough, a father of two, said he tried to help the woman who was shot and yelled for someone to call an ambulance.
"I'm just in awe that something like this could take place. That someone could do such a thing. It's just shocking," Mr McCullough said.
Police pursued the gunman to Reliable Metal Products just north of Geneva, about a dozen miles southeast of Samson, where he fired an estimated 30 rounds from a semi-automatic weapon, the Alabama safety department said.
The gunman then went inside the plant and shot himself, according to the safety department's statement.