Reem Awad, a police spokeswoman, said the teenager, who is from Lorton, Va., was charged with two counts of murder. The department did not release his name because it does not typically identify suspects under 18, she said.

Family members recently tried to persuade Ms. Kuhn-Fricker’s 16-year-old daughter to stop seeing the teenager because they worried he was sharing white supremacist ideas with her, Janet Kuhn, Ms. Kuhn-Fricker’s mother, told The Washington Post.

A few days before her death, Ms. Kuhn-Fricker emailed administrators at her daughter’s high school and attached images of social media posts she attributed to her daughter’s boyfriend that included anti-Semitic and homophobic content and referred to Hitler, Nazi book burnings and white supremacy, The Post reported.

“I would feel a little bad reporting him if his online access was to basically be a normal teen, but he is a monster, and I have no pity for people like that,” Ms. Kuhn-Fricker said in her email to the school. “He made these choices. He is spreading hate.”

Ms. Kuhn said detectives believed Ms. Kuhn-Fricker and Mr. Fricker checked on their daughter on Friday morning and found the 17-year-old in her bedroom, leading to a confrontation that ended in their deaths.