In a letter at once stern and kind, Howard County high school student Ben Vassiliev declared his "monumental" love for his girlfriend and urged a mutual friend to put aside the romantic feelings that he, too, felt for her.

"She's more important to me than anything else ever has been to anyone," Vassiliev wrote to his friend Ryan Furlough, who is now on trial for allegedly fatally poisoning Vassiliev with cyanide. The tension caused by Furlough's feelings for the girl, Vassiliev wrote, had gone "from unpleasant to unbearable."

"I need to know this is over, and I need to know beyond a shadow of a doubt," Vassiliev wrote in June 2002, composing the letter with his girlfriend, Caroline Smith, at his side.

The letter was read aloud yesterday in Howard County Circuit Court on the second day of testimony in Furlough's murder trial. The letter and others like it provided fresh insight yesterday into the tensions among the three Centennial High School students.

Prosecutors say Furlough ordered cyanide over the Internet and then slipped it into his friend's soda in January 2003 as the two played video games. State's Attorney Timothy J. McCrone, in opening remarks Tuesday, said Furlough poisoned Vassiliev, 17, because he was "determined to have" Vassiliev's girlfriend.

Furlough, 19, is charged with first-degree murder. The crime is punishable by life in prison without the chance of parole.

Defense attorneys have not disputed that Furlough poisoned his friend. Rather, they argue that Furlough was depressed and suicidal and that his actions were mitigated by those considerations. Much of the defense case is expected to consist of medical testimony.

The jury will be permitted to consider lesser charges, including second-degree murder and manslaughter, lawyers in the case said.

During most of Tuesday and yesterday morning, Furlough did not visibly react to testimony -- not to an emergency room physician's account of the desperate effort to diagnose Vassiliev's symptoms, and not to a forensic pathologist's clinical description of Vassiliev's autopsy.

He generally has not looked around the courtroom. His eyes have been cast downward, a pen pressed to a pad of yellow paper in his lap, as he has sat nearly motionless for minutes at a time.

And, at first, that did not change when Smith, 18, strode across the courtroom and stepped into the witness box to testify against him. Dressed in all black, Smith, now a math major at the University of Maryland, often looked in the direction of the defense table as she spoke.

She described the friendship the three shared, including the time they went to the junior prom together. She testified that she once kissed Furlough while the two watched the Adam Sandler comedy "Happy Gilmore" and that she felt awful about it later. She testified that Furlough wrote her notes and e-mails, full of love for her and apparent self-loathing.

"I am now determined to win your heart and I will not rest until I have it," one said.

In another, he described himself as a "zombie" and said, "What's the point of killing yourself if you're already dead?"

On cross-examination by defense attorney Jan O'Connor, Smith said Furlough occasionally mentioned suicide. She read a letter she had written to Furlough when he seemed to be emotionally withdrawn in fall 2002. She called him "special" and urged him not to push her away. "It just hurts us both," Smith wrote. "You are my best friend."

Yesterday, Smith testified that she meant those words.

"It wasn't just sympathy," Smith said. "I did like him as a friend."

"He thought Ben didn't care about him, and all I was trying to emphasize was that he did," she said. "It wasn't just me. Other people cared about him, and Ben was one of them."

At that, Furlough dabbed at his eyes with a tissue. His shoulders shook as he pressed his fingers against the bridge of his nose and began to cry.