A jury found 19-year-old Ryan Furlough guilty of first-degree murder yesterday in the poisoning death of a close friend and high school classmate, rejecting a defense claim that an antidepressant led to the slaying that stunned Howard County last year.

The verdict came less than three hours after closing arguments, in which the prosecution described Furlough's plan to kill Ben Vassiliev, 17, by slipping cyanide into his Vanilla Coke, as methodical and deliberate.

Senior Assistant State's Attorney Mary Murphy said the January 2003 slaying was motivated both by Furlough's professed adoration for Vassiliev's girlfriend and by Furlough's sense that his Centennial High School classmate was withdrawing from their friendship, discarding him "like the trash."

In delivering a verdict for first-degree murder, punishable by life in prison without parole, the jury rejected lesser charges that defense lawyers argued were more appropriate given Furlough's "deteriorating" mental condition.

Furlough did not react visibly when the verdict was read. His head was bowed, as it had been during most of the trial.

Once the jury left the room, Vassiliev's father, Walter, stood and grasped the back of the bench in front of him. Supporters stroked his shoulders and whispered to him.

"This is a very emotional time," Walter Vassiliev said later outside of court. "I miss Ben. I miss Ben. This never should have happened."

Susan Furlough, who had testified in her son's defense, said: "Our family is very, very sorry for the tragedy of Ben's family. We loved him like a son, too."

She told reporters she was "deeply upset" by the verdict, which she said leaves adolescents who take antidepressants still "at risk" for the adverse side effects the defense blamed in the slaying.

In his closing argument, defense attorney Joe Murtha offered a picture of the "dark and desperate world" he said Furlough inhabited, describing his client as "a shell of an individual" who was "tortured" by mental illness and self-loathing and who was incapable of "making a clear decision."

"Ryan Furlough sits in front of you as a sunken, shallow, defeated human being," Murtha said.

Furlough was taking what Murtha described as an "excessive dosage" of Effexor, an antidepressant that some experts say can increase suicidal thinking in adolescents.

Murtha cited a Food and Drug Administration warning that the side effects of Effexor might include psychotic depression and a lack of impulse control.

"Something happened to Ryan Furlough that altered his ability to appreciate his own actions," Murtha said.

Murphy, the prosecutor, responded that "the darkness that surrounds the defendant is evil," and she urged the jury to reject the theory that his judgment was impaired. "Many people suffer from depression," she said. "Many people are on antidepressants. They don't go out and kill."

State's Attorney Timothy J. McCrone, who prosecuted with Murphy, echoed that statement after the verdict: "Depression does not justify murder," he said. "The jury demonstrated common sense in coming to that conclusion."

Murtha told reporters that Furlough had prepared himself for a guilty finding on the charge of murder. "His anxiety level has reduced because there's at least closure to this aspect of it," Murtha said.

Since his arrest on Jan. 5, 2003, at the conclusion of a police interview, Furlough has been held without bail at the Howard County Detention Center. Judge Raymond J. Kane Jr. scheduled sentencing for July 20.

Furlough did not testify, but jurors heard from him extensively -- in e-mails and letters, in a forensic reconstruction of his Internet searches and, finally, in a videotaped statement he gave to police starting Jan. 4, 2003, a day after the slaying.

In the videotape, which jurors watched Thursday, Furlough first claimed innocence but then admitted killing Vassiliev, saying he had developed the plan beginning as early as September.

Although he rarely mentioned Caroline Smith, Vassiliev's girlfriend, he complained bitterly of his old friend's failure to give him gifts.

"When there was nothing again and again, I started to think for some reason or other he just doesn't care about me anymore," Furlough said.

"I was trying to forgive him because I didn't have many friends, and he was one of the best that I had."

But, "I wanted to do harm to him," Furlough said. "I wanted to kill him."

Murphy asked jurors to consider the level of planning that went into the poisoning. Furlough, she said, by his own admission developed the plan over several months -- choosing first to poison, then to use cyanide, determining a dosage and finally buying it over the Internet.

"He deliberated," she said. "He made choices at every step of the way."

Ryan Furlough put cyanide into his friend's soda. Ben Vassiliev was friends with his killer.