Authorities expanded their search for the missing pair across the eastern U.S. on Monday.
The girl's sister watched her father get shot to death as he was escorting an 18-year-old man from their home on Sunday after an argument, according to court records.
David G. Ludwig gunned down the missing girl's father and mother and took the girl away from the house at gunpoint, police said. Neither Ludwig nor Kara Beth Borden, who is believed to be Ludwig's girlfriend, has been seen since the slayings, and police are treating her disappearance as a potential kidnapping.
Lititz Police Chief William Seace told a news conference Monday morning that an Amber Alert for the two of them has been expanded to the entire eastern United States.
"We're feeling that because the parents were killed this could be an abduction, but we could be wrong," Seace told reporters Monday.
"It's all over the electronic signs over our interstates here in central Pennsylvania and throughout Pennsylvania, so the chief was very disappointed that nothing came in overnight: No leads, no phone calls," reports Shannon Davidson of CBS affiliate WHP-TV.
Michael F. and Cathryn Lee Borden, both 50, were found shot to death at the family's home in Warwick Township, about 60 miles west of Philadelphia, shortly after 8 a.m. Sunday.
"Carol was brought home by 18-year-old David Ludwig late Saturday night. That was cause for concern to her parents, so they asked he return in the morning," reports Davidson.
The couple's 9-year-old son ran to the home of neighbors, who called 911, police said.
Katelyn Borden, 13, told investigators her father and mother were shot after they argued with Ludwig for about an hour in their home near Lititz on Sunday morning.
"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," according to a police affidavit.