David G. Ludwig killed 14-year-old Kara Beth Borden's parents after they and their daughter argued about her curfew, police said. The girl was last seen Sunday morning at the family's home in Warwick Township, about 60 miles west of Philadelphia.
"We don't know whether she has been abducted or is willingly a part of this," said Police Chief William Seace. Until they can determine otherwise, police are operating on the assumption that she has been kidnapped.
Borden's parents, Michael and Cathryn, both 50, were found shot to death shortly after 8 a.m., after the couple's 9-year-old son ran to the home of neighbors for help. An older daughter also escaped.
"The parents really didn't like them hanging out," family friend Stephanie Mannon, 16, said. "There was a big difference so that's kind of understandable. She's 14 and he was 18."
"(Cathryn) was concerned that Kara was boy crazy like many young teens, but she was e-mailing back and forth and she poured her heart out to me, 'What should I do,' and 'What a situation,' you know, trying to avoid tragedy," family friend Reba Zimmerman said on CBS News' The Early Show.
Police late Sunday issued an arrest warrant for Ludwig on two counts of criminal homicide, one count of recklessly endangering and one count of kidnapping. They also issued an Amber Child Abduction Alert for the girl.
Seace said authorities do not know which direction Ludwig was heading and the search area for him and Kara was "anywhere in the state of Pennsylvania or along the eastern seaboard."
"We think they're boyfriend and girlfriend," said Seace. "The young girl was out during the night, came home, and her parents confronted her. From what we understand, he came to the house."
"I think in this community everyone is very shocked, simply knowing the Bordens as a kind, loving family," Zimmerman told Early Show co-anchor Julie Chen. "In this community, nothing like this ever happens."
But the murders were the second violent incident in a week in normally quiet Lititz, a Lancaster County village known for its quaint shops, local artists and such attractions as the Sturgis Pretzel House, which bills itself as America's first pretzel bakery.
On Tuesday, police shot and killed a man hours after authorities said the suspect shot one of three officers who had gone to his home with a traffic warrant.
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.
Neighbors say Kara Beth and Ludwig were both home schooled and knew each other through a home school network, reports CBS News correspondent Sharyn Alfonsi.
"I know he's a Christian and he seemed like a really decent, nice sweet person," said Mannon. "I worked with him. I can't believe it happened."
Neighbor John Hohman, 40, said his family got a phone call from Lancaster County emergency management officials Sunday morning warning them to stay inside. He said he looked out a window and saw police running through the neighborhood. About an hour and a half later, authorities told residents to go to their basements.
"We were really upset. We didn't know what was going on," Hohman said.