David Ludwig, 18, and Kara Beth Borden were taken into custody around midday after he crashed his parents' car head-on into a tree in Belleville, some 600 miles from where the killings took place.
Investigators said it was not immediately clear whether the girl was abducted or went along willingly, but they were operating on the assumption she was kidnapped. Police said they were questioning Ludwig, but since Borden is a minor, she has not yet been questioned.
"It's completely insane, completely insane," Lancaster County, Pa., Coroner G. Gary Kirchner said. "This isn't a Romeo-and-Juliet deal. This is far worse than that."
Authorities said Ludwig shot Kara's parents, Michael F. and Cathryn Lee Borden, early Sunday after they and their daughter argued about her curfew when she came home late. The shootings happened at the family's home near Lititz, Pa., about 60 miles west of Philadelphia.
An alert for the girl was issued across the East, and police in Pennsylvania and Indiana investigated reported sightings of the pair as they made their way west in a red Volkswagen Jetta. They were stopped in Belleville, about 20 miles west of Indianapolis.
The two had been spotted at a central Pennsylvania gas station Monday morning, but were not caught then.
The shift manager at a truck stop Subway shop in Lamar, about 90 miles from Borden's home, said she sold the couple an Italian sub at about 9 a.m. on Monday, and thought they matched a radio description she had heard earlier.
"Her face was all puffy, her eyes were puffy," Amanda Workman told The Associated Press. "You could tell she was crying."
Workman said the young couple paid cash and had apparently made another purchase from a different part of the truck stop.
"They were really shady. They didn't want to talk a lot. They just ordered their sandwich and left," Workman said.
Workman said the photograph police showed her of Kara Borden matched closely, and recalled that she was wearing a black hooded sweatshirt with a printed pattern. Ludwig's hair is now much shorter.
"The guy that I seen had short hair, like he had shaved his head," Workman said.