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An 18-year-old Kentucky man and his 13-year-old girlfriend who have been missing for 12 days are believed to have taken off on a crime spree across the South, authorities said Thursday, during which they're suspected of having stolen at least two vehicles — one of which had guns in it.

"It is imperative that these two be located and apprehended as their behavior is becoming increasingly brazen and dangerous," the Grayson County Sheriff's Office said in a statement.

The sheriff's office identified the pair as Dalton Hayes, 18, and his girlfriend, Cheyenne Phillips, 13, whom Cheyenne's father reported missing on Jan. 3. They're accused of stealing a neighbor's red Toyota pickup truck, which was spotted on security video nine days later outside a Walmart store in Manning, South Carolina. The couple themselves were captured on video entering the store.

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They apparently ditched the truck Thursday in Henry County, Georgia, south of Atlanta, and are believed to have stolen a silver Toyota Tundra — which Grayson County Sheriff Norman Chaffins said had .45- and .38-caliber handguns in the back seat.

Chaffins said that both are suspects in at least the two auto thefts and that Hayes is also wanted on charges of custodial interference — that is, luring Cheyenne, a minor, away from her legal guardians.

"We're hoping the two of them have enough sense not to do something with that gun and hoping it never comes to that," Chaffins told NBC station WAVE of Louisville.

Hayes' mother, Tammy Martin, told reporters that her son had just recently started dating Cheyenne, who she didn't know was only 13. She said Hayes texted her on Jan. 6 to say, "Mommy, don't worry. I'm fine, okay — plenty of money and food. Love you, good night, sweet dreams."

"Whatever it is, it can be worked through," Chaffins said. "That's what we want to tell these young people — we want them to come home."

Dalton Hayes, 18, and Cheyenne Phillips,13, in photos from the Grayson County, Kentucky, Sheriff's Office. Grayson County, Kentucky, Sheriff's Office

SOCIAL

@Rene_gadeCowboy need your help Police in KY looking for missing 13 y/o Cheyenne Phillips & 18 y/o Dalton Hayes pic.twitter.com/SFWYFdBKWF — Sayreville Truth (@SayrevilleT) January 14, 2015

18yo Dalton Hayes' mother says she didn't know Cheyenne Phillips is 13. She says the couple took off when she confronted them about her age — Karma Dickerson WHAS (@WHAS11karma) January 14, 2015

— M. Alex Johnson