Laurel County Sheriff's Commander Richard Dalrymple said at Sunday night's news briefing that Couch purchased about a thousand rounds of ammunition, most of which has been recovered, ABC News reported.
The suspect remains at large despite a massive search in the area of Saturday's shooting north of London, Kentucky.
The shooting unfolded around 5:30 p.m. local time on Saturday evening, officials said. Arriving deputies initially found nine vehicles had been shot in both the north and southbound lanes of I-75, Laurel County Sheriff John Root said at a news conference late Saturday night. By Sunday night, that number had jumped to 12 vehicles, according to officials.
Officials update on ongoing search for shooter along I-75 in Kentucky
Root said deputies found five people with serious gunshot wounds, including one who was shot in the face. He said one vehicle contained two people who were shot.
Laurel County Sheriff's Deputy Gilbert Acciardo said Sunday that none of the victims suffered life-threatening injuries and were all in stable condition.
"A couple of our deputies, because of the severity of the injuries, loaded the people up, the injured persons, and transported them to London Hospital," Root said.
Two additional people were injured in a car crash that occurred during the shooting, authorities said.
Root said I-75 was immediately shut down in both directions, saying that at the time, deputies didn't know where the bullets came from.
"We couldn't risk somebody else being shot," Root said.
Authorities previously said they found an AR-15 rifle in the woods near the crime scene on Interstate 75, about eight miles north of London, Kentucky. Couch's vehicle was also found abandoned in the same area Saturday night, officials said Sunday afternoon.
Couch was first named as a person of interest in the incident but upgraded to a suspect on Sunday. Root said the decision to name Couch a suspect was based on evidence collected in the investigation. Asked to elaborate, Root said that the recovery of the weapon and Couch's vehicle, as well as "some information" he could not share, prompted investigators to elevate Couch to a suspect in the shooting.
The sheriff's office earlier released a photo of Couch, who allegedly fled the freeway shooting and was believed to still be in the area, Root said.
Root said Couch has an address in Woodbine, Kentucky, and the sheriff's office described him as about 5-foot-10-inches tall and 154 pounds.
Dalrymple said Sunday night that the suspect allegedly fired from a ledge about 30 feet down from a cliff by Exit 49. To find the location, Dalrymple said he had to hold onto a tree and look down to see the site to find the location.
Earlier, Acciardo described the shooting as "sniper-like" and said it was not the result of road rage. He said investigators do not believe the shooter knew any of the victims or had contact with them before the shooting.
Up to 60 members of law enforcement searched the area of the shooting until 3 a.m. Sunday before halting the search due to safety concerns, saying it was pitch black on the highway and describing the terrain where the search was being conducted as very rugged.
More than 150 individuals were involved in the search for Couch on Sunday, with efforts set to resume on Monday morning, according to Root.
The FBI, the U.S. Marshal's Service and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives are assisting local authorities in the investigation, officials said.
A motive for the shooting remained under investigation.
Kentucky Gov. Andy Beshear said in a statement on X on Saturday that he was monitoring the situation.
In an interview Sunday on ABC's "Good Morning America," Christina Dinoto said she was driving with a friend southbound on I-75, heading to Tennessee, when the shooting erupted.
"All of a sudden, we just heard this loud, deafening sound," Dinoto said. "And my ear, my right ear, started ringing, and we didn't know what the sound was, but we both looked at each other and said, was that a gunshot?"
Dinoto said that when she pulled off the interstate in Knoxville, she discovered damage to her vehicle that she suspects was caused by a bullet that may have ricocheted off another car.
The Kentucky shooting came less than a week after six people were injured in six shootings that occurred on Sept. 2 on Interstate 5 in Washington state between 8:26 p.m. and 11:01 p.m. local time, officials previously said. A suspect whose vehicle was sought in connection with several of the shootings was arrested in the Tacoma area on Sept. 3, police said.
ABC News' Alex Presha contributed to this report.