No weapons were found in the car. Police did not know why the pair were in Indiana.
Pennsylvania authorities were traveling to Indiana. Prosecutors expected to hold an extradition hearing for Ludwig on Tuesday or Wednesday, unless he waives those proceedings.
Kara's 13-year-old sister, Katelyn, told investigators her parents were shot after they argued with Ludwig for about an hour, according to court papers.
Katelyn said she saw Ludwig shoot her father, and then ran into the bathroom, where she heard a second shot, presumably the one that killed her mother, court papers said. Ludwig then ran through the house calling for Kara, she told investigators.
The couple's 9-year-old son ran to the neighbors, who called 911.
Police late Sunday issued an arrest warrant for Ludwig on charges of criminal homicide and kidnapping.
Stephanie Mannon, 16, said Ludwig and Kara 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 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."
The Bordens, both 50, were apparently shot once each in the head, authorities said. Mike Borden worked for a printing company, and the children were home-schooled, said neighbor Tod Sherman. Sherman said the family knew Ludwig through a home-schooling network.
It was the second violent episode in a week in normally quiet Lititz, a Lancaster County village known for quaint shops, artists and Sturgis Pretzel House, which bills itself as America's first pretzel bakery. On Tuesday, police shot and killed a 23-year-old man after he shot and wounded an officer who had gone to his house to arrest him on a traffic charge.
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On the Net:
Kara Borden's blog: http://www.xanga.com/karebear000
David Ludwig's blog: http://www.xanga.com/Haydren
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Associated Press writers Rick Callahan in Indianapolis, Martha Raffaele in Lititz, Pa., and Mark Scolforo in Harrisburg, Pa., contributed to this report.
