Rockville High School senior Michelle Miller had “huge plans” for a future in the Army Reserve and the 17-year-old honor student was set to begin basic training this summer, her father said.

On Monday, she was found dead in the Germantown home of a U.S. Army staff sergeant who, her family said, had helped recruit her. Montgomery County police said 31-year-old Adam A. Arndt shot Miller and then killed himself, leaving her family, friends and classmates to mourn a bright, ambitious teen who was only months from graduation.

“She wanted to be a psychologist, and she wanted to do battle-related psychology,” said Michelle’s father, Kevin Miller. “She was a beautiful girl, and she loved life.”

Miller said Michelle played varsity lacrosse and soccer and tutored other students. She took Advanced Placement courses and was taking a class at Montgomery Community College, he said.

Will Minger, 19, a Rockville High School graduate who is now at East Carolina University, said Michelle was bubbly and outgoing. When he was in high school, he said, she was part of the cheering section for the basketball team and was a water girl for the football team.

“She carried a smile with her everywhere she went, and her smile was infectious,” Minger said in an e-mail. “There was never a dull moment when you were around Michelle, and that’s not something you can say about most people.”

Police said Michelle Miller and Arndt had met through her plans to join the Army Reserve. They did not say what they thought prompted the shooting.

Kevin Miller said he thinks Arndt was a higher-ranking colleague of a female Army recruiter who first spoke to the family. He said that he never met Arndt, but that Michelle talked about him.

“I know she was smitten by the guy — she used to show a picture of him on her camera,” Miller said. He did not know whether the two were romantically involved but said, “I’d say the chances are pretty good.” He said Michelle rushed out of their house about 9 p.m. Sunday, telling her parents that she had to help a friend who was suicidal, but didn’t give many details. Her father said that he sent text messages and that when she didn’t respond he called police.

“She gave me really vague directions, the location where it was, and I never heard from her again,” he said.

Detectives learned that Michelle might be at Arndt’s Pinnacle Drive townhouse. Just after 10 a.m. Monday, police forced their way in and found the two dead. A relative of Arndt’s declined to comment when reached by phone. Army officials were not immediately available to comment.

As friends and classmates learned of Michelle’s death, social media sites filled with tributes and memories. “I already miss her so much,” one wrote, and another called her a “second sister.” “Wishing I could do something,” another wrote, “but I can’t.”

In a letter to parents, Rockville High Principal Debra S. Munk described Michelle as “an active member of the Senior Class.” She said psychologists and counselors would be available to help classmates. “For those of you who knew Michelle,” Munk wrote, “we ask that you remember and celebrate her life.”

Jennifer Jenkins and Clarence Williams contributed to this report.