US prosecutors seek death penalty for man involved in 'calculated' murder of Chinese student

US prosecutors argued on Monday that an Illinois man who raped and murdered a Chinese graduate student two years ago should be executed, and called on her heartbroken fiance and friends to tell the jury about the victim's kind, optimistic nature.

A federal jury in Peoria, Illinois found Brendt Christensen, 29, guilty last month of the abduction and murder of Yingying Zhang, a 26-year-old student at the University of Illinois at Urbana-Champaign. The jury is now in the penalty phase of the trial.

"This was not an ordinary crime," James Nelson, a prosecutor in the US Department of Justice's capital case division, told the jury. "It was cold, cruel and calculated."

Christensen's lawyers have asked the jury to spare his life, saying he had long struggled with substance abuse and mental illness and had a family history of both.

"No one who grew up with Brendt would have ever guessed that this is how his life would end up," Julie Brain, one of his lawyers, told the jury. "This was a man secretly struggling with mental health his whole life."

She showed the jury photographs of Christensen dressed in his Boy Scouts and football uniforms.

The jury is being asked to decide between the death penalty and life in prison without the possibility of parole.

The victim's fiance, Xiaolin Hou, told the jury the couple met while undergraduates at Sun Yat-sen University in Guangzhou, China.

"She's smart, she's brave, she's optimistic, she's the best girl I ever met," he said.

Videotaped testimony by Zhang's college friends was also played to the jury. Christensen, dressed in gray pants and a gray shirt, watched the videos alongside his lawyers.

Lixha Fang said she worried for her friend's safety when she moved to the United States because Zhang was so nice to everyone.

Another friend, Ye Cai, said when she first met Zhang in their dorm, she was struck by a "skinny, very smiley girl," and the pair soon became close friends. She recalled Zhang was very close to her family and called them frequently. Hearing this, Zhang's brother got up and left the courtroom in tears.

Cai also read aloud some of the many messages the pair swapped over WeChat, the popular Chinese social media platform.

"How big is the world?" Zhang wrote in one of the messages. "I will measure it with my feet."

Federal prosecutors are seeking the death penalty under US kidnapping laws.

Zhang was reported missing on June 9, 2017, two months after coming from southeastern China to study photosynthesis and crop production at the university. Her remains have never been found, but prosecutors said her DNA was matched to blood later found in three spots inside Christensen's bedroom.

Investigators were led to Christensen through surveillance video footage captured in Urbana, 210 km south of Chicago, that showed Zhang getting into a black car that later was traced to Christensen.

Prosecutors said Christensen, a one-time master's student at the university, took Zhang to his apartment, where she fought for her life as he bludgeoned her with a baseball bat, raped her and stabbed her in the neck before cutting off her head.

Earlier in the trial in US District Court in Peoria, prosecutors characterised Christensen as having a fascination with serial killers, including Ted Bundy, who murdered dozens of women during the 1970s and was put to death in 1989.

Details of the crime, including Zhang's decapitation, were revealed by Christensen himself in conversations with his then-girlfriend secretly recorded for FBI agents investigating the case before his arrest, according to trial testimony.