CHRISTIANSBURG, Va. - A former Virginia Tech graduate student pleaded guilty yesterday to decapitating a classmate, and prosecutors for the first time revealed the reason for the campus cafe killing, saying that his romantic advances had been rejected.

Prosecutors described in detail a heartbroken Haiyang Zhu who had fallen in love with Xin Yang, only to be rebuffed when she told him that she had a boyfriend whom she planned to marry. Previously, authorities had been tightlipped about what led to the killing.

Zhu, who faces up to life in prison, did not say at the plea hearing why he had killed Yang. He quietly anwered questions in English.

"Your honor, I plead guilty," he said, his hands shackled to a chain circling his waist.

Prosecutor Brad Finch said that on the morning of the killing, Jan. 21, Zhu bought the 8-inch butcher knife used in the murder, two other knives and a claw hammer. He also called the 22-year-old woman a dozen times after buying the weapons.

Finch cited a letter that Zhu wrote while in jail that said that Yang's rejection "forced him to kill her" because "he loved her too much."

"Xin broke his heart on the morning of January 20th when she told him that she had a boyfriend and that they planned to get married," the letter said, according to Finch.

The killing stunned a campus that still had vivid memories of the mass slayings in April 2007, when a student gunman shot 32 people and then took his own life. The stabbing was the first slaying on campus since then.

Zhu's first-degree murder plea did not qualify for the death penalty under Virginia law, but Finch said that he would seek the maximum penalty. Virginia does not have parole.

Before the killing, Zhu penned what Finch termed a love letter that was found in her dorm room. The letter was written shortly after Zhu first met her and indicated he had "fallen deeply in love."

"She makes him happy and fulfilled, that she is beautiful and that he will treasure her forever," Finch said. "The defendant asked Xin to be his girlfriend."

Finch also described the attack in detail, noting that Yang suffered numerous defensive wounds to her hands and arms as she tried to fend off Zhu. She eventually fell and he severed her head. He was holding it when police arrived.

About seven other people who were in the shop at the time told police that the two hadn't been arguing before the attack.

It appeared that Yang, who was from Beijing, had met Zhu, of Ningbo, China, only recently, Kim Beisecker, director of Cranwell International Center, has said. Zhu, a doctoral student in agricultural and applied economics, had been assisting Yang in adjusting to life at Tech, something the 500 Chinese students often do for new members in their community.

Montgomery Circuit Judge Robert Turk said that Zhu will be sentenced on April 19.