Indiana State Police say they have captured the man wanted in the Pennsylvania double homicide; a girl with him is unharmed.
Earlier, police said David G. Ludwig and Kara Beth Borden were spotted at a central Pennsylvania truck stop.
A worker at the truck stop told The Associated Press the girl appeared to have been crying.
Police said David G. Ludwig killed 14-year-old Kara Beth Borden's parents, Michael F. and Cathryn Lee Borden, early Sunday after they and their daughter argued about her curfew. The shootings took place at the family's home in Warwick Township, about 60 miles west of Philadelphia.
Lititz Police Chief William Seace said Monday afternoon that Ludwig and Kara Beth, who is believed to be Ludwig's girlfriend, had been "positively identified" at a truck stop in Lamar, about 90 miles from the Borden home.
Amanda Workman, the shift manager at the Subway sandwich shop at the truck stop, said she sold the couple an Italian sub at about 9 a.m. Monday, and thought they matched a radio description she had heard earlier.
"Her face was all puffy, her eyes were puffy," Workman told The Associated Press. "You could tell she was crying."
Workman called police right away, first to get a better description and then to report a possible sighting. It took police about 20 minutes to respond, she said.
Officials have said that until they can determine otherwise, police are operating on the assumption that Kara Beth has been kidnapped.
An Amber Alert for the two of them has been expanded to the entire eastern United States, Seace said Monday.
Kara Beth's 13-year-old sister, Katelyn, told investigators her father and mother were shot after they argued with Ludwig for about an hour, according to a police affidavit filed in court.
"As they got near the front door, Katelyn Borden saw David Ludwig with a handgun pointed toward her father and Katelyn saw David Ludwig pull the trigger, heard a gunshot, and then she ran into the bathroom," the affidavit said.
She heard a second shot -- presumably the one that killed her mother -- while hiding in the bathroom, it said. Ludwig then ran through the house calling for Kara, she told investigators.
The couple's 9-year-old son ran to the home of neighbors, who called 911, police said earlier.
Police late Sunday issued an arrest warrant for Ludwig on two counts of criminal homicide, one count of reckless endangerment and one count of kidnapping. He was last seen driving his parents' red Volkswagen Jetta.
"We think they're boyfriend and girlfriend," said Seace. "The young girl was out during the night, came home, and her parents confronted her."
Lancaster County Coroner G. Gary Kirchner said Monday that the victims, who were both 50, apparently died of single gunshot wounds to the head. Autopsies were being conducted.
"It's completely insane, completely insane," Kirchner said. "This isn't a Romeo-and-Juliet deal; this is far worse than that."
It was the second violent incident in a week in normally quiet Lititz, a Lancaster County village known for quaint shops, local artists and Sturgis Pretzel House, which bills itself as America's first pretzel bakery.
On Tuesday, police shot and killed 23-year-old Daniel B. Faust hours after he allegedly shot one of three officers who had gone to his home with a traffic warrant. The officer, Jevon M. Miller, was critically wounded.
The Borden family had lived in their home for several years, said neighbor Tod Sherman, 47. Mike Borden worked for a printing company, and the children were home-schooled, he said.
Sherman said the family knew the 18-year-old suspect through a home-schooling network.
Stephanie Mannon, 16, said Ludwig and Kara Borden had been seeing each other secretly.
"Their parents didn't approve of them being together" because of the age difference, she said. "It wasn't because he was a shady character, because he wasn't."
Both Ludwig and Kara Borden maintain Web sites. Hers refers to interests in soccer, art and her Christian faith; his says he enjoys "having soft air gun wars" and claims expertise in "getting in trouble."
Neighbor John Hohman, 40, said his family got a phone call from officials Sunday morning warning them to stay inside. He said he looked out a window and saw police running through the neighborhood. Later, authorities told residents to go to their basements.