Police said that Arnold and Bulmer, who had blood on his hands and clothes, were found sitting in an upstairs bedroom when officials arrived and found Lane dead in another bedroom. Another adult resident had heard a commotion and called 911.
The killing astonished residents of the close-knit community and those who said the father and daughter were devoted to each other. The scenario described by police was equally unfathomable: Arnold encouraged Bulmer to stab her father and told him that they would run away together afterward.
But a motive remains unclear, police said.
“We really at this point don’t know why they wanted to hurt or kill Mr. Lane,” said Sherry Llewellyn, a Howard County police spokeswoman. “We have not been able to establish a motive. We’ve just gathered evidence that points to them being responsible.”
According to police, they decided to carry out the crime on Thursday after discussing it through “electronic communication” for the past two months. Arnold left open a sliding glass door, allowing Bulmer to slip in around 4 a.m., police said.
Lane worked in commercial real estate and was well known through his blog, “Tales of Two Cities,” where he wrote about real estate and other county issues and often referred to his daughter as “peanut.” He also hosted a podcast.
Baltimore City Councilman James Kraft, a friend, called Lane “one of the best people I know — everybody loved the man.”
“Morgan was his life,” Kraft said. “He loved that girl. He just loved that girl. . . . Everything I know about Morgan and Dennis is that they had a great relationship. . . . My whole knowledge is that she loved her father and he loved her.”
Relatives of Bulmer and Arnold could not be reached or declined to comment Friday.
Bulmer and Arnold posted frequently on social media. The pair often wrote to each other, expressing their mutual affection.
Clay Bleyer, 14, also a freshman at Mount Hebron, said Arnold and Bulmer hung out mostly with each other — they “just did their own thing” — and he often saw them laughing together.
“The only person I saw her with in the halls was with Jason,” he said. “They would always hold hands in the hallway, and Jason would always wait outsider her classes for her.”
Lane had joint custody of Arnold with her mother, an arragement worked out in court in 2009, records show. Bill Harrison, who worked with Lane, said Arnold would live with Lane during the week and stay with her mother on the weekends. Harrison also said that Lane was engaged to be married and that he owned the home with his fiancee.
A woman at the home Friday declined to comment, asking for privacy.
Howard County Council member Courtney Watson said Lane was a friend to “many, many people in the Howard County community.”
“It’s going to take some time to make sense of it, if one can make sense of it,” she said.
Sun reporters Jessica Anderson, Ian Duncan and Pamela Wood contributed to this report.