A D.C. drug dealer who prosecutors say sold the heroin that killed a 16-year-old McLean High School student was sentenced Friday to 25 years in prison — a stiff term that a federal judge said was necessary because the man had an extensive criminal history and sold drugs to young, vulnerable users.

As she handed down the sentence, U.S. District Judge Leonie M. Brinkema told the 37-year-old Antowan Thorne that he seemed to have “absolutely no potential of being a law-abiding citizen,” at least not now. Prosecutors had asked for precisely the term Brinkema imposed, saying that although Thorne was not convicted directly in connection with Emy­lee Lonczak’s death in August 2013, there was no doubt that she died after using heroin that he sold.

“Miss Lonczak’s punishment did not fit her crime, your honor,” Assistant U.S. Attorney Michael Ben’Ary told Brinkema. “But we ask that the defendant’s fit his.”

Thorne declined to say anything on his own behalf. His attorney, Gregory E. Stambaugh, said in court that his client “does want to move on with his life, as anybody would.” He had asked in court filings that Thorne, who has a 19-year-old son and worked most recently for a lawn-care company, be sentenced to five years in prison — the mandatory minimum term.

The case garnered national attention, epitomizing what federal authorities termed a frightening uptick in young people using heroin. The circumstances of Lonczak’s death, too, made it especially notable.

Federal authorities said Lonczak — a quiet girl who played soccer and was interested in fashion design — had never injected heroin before the night she died. After buying the drug from Thorne in the District, she used it and passed out in a car. A friend took her to his house, and when he saw she was dead the next morning, he discarded her body in bushes and covered it with a window screen.

The friend, Kyle Alifom, was sentenced to six years and eight months in prison earlier this year.

Prosecutors had alleged that Thorne was the source of the heroin that — combined with a common antihistamine — caused Lonczak’s death.

Thorne, though, was convicted in August only of a drug distribution charge; Brinkema ruled that prosecutors had not produced enough evidence to hold him responsible for the death. At Friday’s sentencing, Brinkema was much more receptive to their arguments, agreeing that Thorne had been involved in criminal activity essentially since he was a teenager and was likely to re-offend.

“You appear to be unable to control yourself,” Brinkema told Thorne.

Prosecutors had asked Brinkema to consider how Lonczak’s death had disrupted her family’s life and how Thorne sold his drugs to youths in Northern Virginia. Ben’Ary said Thorne considered them the perfect customers because they were mobile, had access to money and were “too young to realize the consequences of their actions.”

Ben’Ary said that although no sentence could bring Lonczak back, Brinkema could “make sure the defendant pays for the choices he made.”

“Miss Lonczak certainly has,” he said.

Lonczak’s mother, who sat in the back of the courtroom but did not speak at the sentencing, declined to comment afterward.

Stambaugh told the judge in court that he planned to appeal his client’s conviction, although Brinkema noted that he might want to withdraw from the case because of disagreements he had with his client. Stambaugh largely declined to comment afterward, noting the judge’s comments.

“It’s just a sad situation all the way around,” he said.