Five people were injured and bullets hit 12 vehicles after shots were fired at traffic on Interstate 75 (I-75) in southeast Kentucky at around 5.30pm local time on Saturday.

The attack happened near London, a town of around 8,000 people in a rural part of the state, about 75 miles (120 kilometers) south of the state capital, Lexington.

Police named former National Guard serviceman Joseph Couch, whom they had originally identified as a person of interest, as a suspect.

Officers found an SUV he owned on a service road near the crime scene and later found a semi-automatic rifle they believed was used in the shooting in "a location that you could have shot down upon the interstate from", Sheriff's Deputy Gilbert Acciardo said.

With the gunman still at large, more than a dozen local school districts and private schools, including those in Laurel, Jackson, and Clay Counties, cancelled Monday's classes, Lex18, a news station affiliated with NBC News, said.

London Mayor Randle Weddle urged locals to "lock doors, keep porch lights on, and monitor security cameras".

State troopers from across Kentucky have been brought in to help search a rugged, hilly area around eight miles from the town.

Spokesman Master Trooper Scottie Pennington said combing the search is like "walking in a jungle with machetes needed to cut through thickets of woods".

The search was temporarily suspended once it became dark on Sunday night but was set to resume on Monday morning.

Laurel County Sheriff John Root said on Sunday: "We're not going to quit until we do lay hands on him. We're doing everything that we can do. Just be confident."

Deputy Acciardo said: "As this continues, it becomes more stressful for the community, it becomes more stressful for the officers that are there because we're looking ... and we're trying to find him, and we haven't found him."

Mr Acciardo said it appears that the attacker planned the shooting for that location because it is very remote and the terrain is hilly, rocky, and hard to navigate.

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Couch, 32, bought the weapon and about 1,000 rounds of ammunition on Saturday morning in London, authorities said.

Couch has a military background, having served in the National Guard for at least four years, Captain Richard Dalrymple of the Laurel County Sheriff's Office said.