Four of the shootings — three at vehicles and one at the school last month — were from the same gun, Franklin County Sheriff's Chief Deputy Steve Martin said Tuesday.
Although ballistics tests could not link the rest of the shootings along Interstate 270, investigators said they "are comfortable" saying all 12 are connected, he said. He would not elaborate.
"To date over 225 leads have been called into the Franklin County Sheriff tip line," Martin told CBS affiliate WBNS at a Columbus press conference.
"Law enforcement is following those leads as quickly as possible. We continue to encourage the public to call in any tips no matter how insignificant it appears."
The shootings began in May along Interstate 270, the freeway that circles Columbus. Many were not reported until after Nov. 25, when 62-year-old Gail Knisley was killed by a bullet that pierced the side of a car driven by a friend.
The latest shooting linked to the spree was a Nov. 11 shooting at Hamilton Township Elementary School in Obetz, about two miles from the freeway.
Superintendent Bill Wittman said he believes the shooting was not meant to harm anyone because it happened overnight, but nervous parents expressed concern.
Tiffany Ellis, 32, said her son's second-grade classroom faces the front of the school, where the bullet struck.
"It makes me angry to be honest with you, that I have to drive down the road worrying about getting shot," Ellis said Tuesday.
She said she plans to call Wednesday to see what precautions the district is taking, and may avoid her own living room, which also faces the two-lane road.
"That's kind of scary to think someone could shoot through your window like that," Ellis said.
A house was shot at Tuesday near the freeway, but Martin said investigators have not linked it to the other shootings.
Local businesses have established a $10,000 reward for information leading to an arrest.
Criminal behavior experts have varying opinions on who's behind the attacks. Jack Levin, a Northeastern University criminologist, believes two people could be responsible.
"When I see a crime like this it's almost always two friends who probably wouldn't do this separately, but when they're together there's a certain chemistry, a certain insanity," said Levin, director of Northeastern's Brudnick Center on Violence.
"How do you share the joy of killing or causing problems if you're alone? You can't brag about it or someone will turn you in," he said.
But N.G. Berrill, a psychologist who profiles killers at his New York forensic consulting firm, said the shooter is probably a young male who feels frustrated and generally powerless in his life.
"It's almost an infantile rage is the way I would describe it," he said.
Berrill said the shootings were the work of a person who loves the thrill of causing panic, although he may not intend to kill.
The shooter is likely someone from the surrounding community, said Lou Palumbo, a retired police investigator who runs a private security and investigation organization called The Elite Agency Limited.
"He didn't have to travel far. He's got a certain comfort level with that landscape which could be what's allowing him to be drawn back to the same location of the shootings," Palumbo said.
Martin, the chief deputy, said investigators are not relying on a profile and are exploring all possibilities.
Shooting at a moving vehicle on a freeway could be a game to the person holding the gun, an expert told the Columbus Dispatch.
"Because the victim is inside a car, it can be psychologically easier for the shooter," said James Alan Fox, a professor of criminal justice at Northeastern University in Boston and a former FBI official. "In their mind, they're shooting at inanimate objects. It's more like target practice or a video game."
He compared freeway shooters to those who drop rocks and other objects from overpasses onto moving vehicles.
"The fact that someone is inside the car certainly raises the stakes," he said. "It adds to the excitement and the risk."
However, "if this started as someone looking for thrills, things have changed. This is murder."