The 19-year-old man charged in the fatal stabbing of Dennis Lane allegedly told investigators that his girlfriend had instructed him to kill her father and his fiancee, specifying the number of times each was to be stabbed in the throat — 10 for him and 15 for her.

In a conversation at school hours before the Ellicott City blogger and businessman was killed, Jason Anthony Bulmer said 14-year-old Morgan Lane Arnold told him: “I don’t know what I’ll do if you don’t do it tonight,” according to charging documents released Monday.

Bulmer sent her a picture of a kitchen carving knife before he entered through a sliding glass door that she had left unlocked, police said. “Arnold gave a reply that indicated to Bulmer that this knife was satisfactory,” Detective Donald Guevara wrote in the court papers.

The documents offer the first indication that fiancee Denise Geiger also was targeted in the attack early Friday on Lane, which shocked residents and those who knew Lane and his daughter. Arnold and Bulmer are each charged with murder, and made their first court appearances Monday. Both will continue to be held without bail.

According to court records, Arnold had a rationale for the way Lane and Geiger were to be killed. “I have my reasons and I’ll tell you later,” she allegedly told Bulmer, who relayed the conversation to investigators. Afterward, Arnold promised the pair would run away to California, Bulmer allegedly told police.

But Geiger survived, and the charges don’t indicate that she was harmed. She called police from the Ellicott City home she owned with Lane and reported that her fiance was struggling with Bulmer. By the time police arrived, according to the documents, Lane was dead in one room; Bulmer and Arnold were in another.

The charging documents reference a string of communications between the two students in the days and hours before the killing of Lane, a prominent community member in Howard County.

Police said Bulmer gave them an account of the plot after he was arrested. Bulmer told police that he had killed Lane and that Arnold had told him to do it, according to the documents.

Both Mount Hebron High School students appeared separately in court Monday via video link.

Arnold’s mother, Cindi Arnold, was in the courtroom but declined to speak to reporters. In a previous statement, she described her daughter as a “special-needs child.”

Gary Bulmer, Jason’s father, who lives in Plymouth, Mich., and is divorced from the teenager’s mother, described his son in an interview as a gentle young man who had trouble with school work.

“He’s a good kid, but he’s slow in school and can’t finish high school,” said the father. “He gets along fine with other kids.”

— Baltimore Sun

Michael Dresser contributed to this report.