A 2017 file photo shows a memorial at the site where University of Illinois at Urbana-Champaign student Zhang Yingying was last seen, in Urbana in 2017. [Photo/Agencies]

A federal prosecutor on Monday called for a jury to sentence to death a former University of Illinois student convicted of kidnapping and killing a Chinese scholar two years ago, calling the crime "cold, calculated, cruel and months in the making".

In his opening statement, Assistant US Attorney James Nelson told the jury in US District Court in Peoria, Illinois, about convicted murderer Brendt Christensen's brutal slaying of Zhang Yingying and his obstruction of the investigation by cleaning the crime scene and hiding her body.

Monday was the first day of the trial's sentencing phase.

The jury that found Christensen guilty on June 24 will decide whether he should be executed after the penalty phase, which is expected to last a few days. Christensen faces a possible death penalty because he was charged under federal statutes. The state of Illinois abolished its death penalty in 2011.

Nelson described the hurt that the family felt by describing Zhang as "far more than just an international scholar". She was "a devoted and loving daughter" who was "the hope of her family", reported The News-Gazette of Champaign, Illinois.

However, after Christensen killed her and disposed of her body, "there will be no proper burial in China. There will be no closure. You will see the anguish," said Nelson, who added that the crime "deserves an extraordinary penalty".

Defense attorney Julie Brain, in her opening statement, encouraged jurors to make a moral decision and to keep an open mind. While admitting Christensen's guilt, she said "this is not a case that deserves the death penalty".

Brain highlighted Christensen's mental health struggles and refuted the argument that he is a serial killer.

She encouraged the jury to sentence him to life in prison without the possibility of release, saying that way he "will die in prison, alone, with strangers".

A photo of Yingying Zhang released by the police. [Photo/police.illinois.edu]

She also pointed out that he has shown no signs of dangerousness in two years in jail.

Prior to the opening statements from both sides, US District Court Judge James Shadid decided on evidence that will be admitted in the penalty phase of the trial, including seven video clips made by Zhang's mother and friends of the young woman, who went missing from the university campus in Urbana, Illinois, in June 2017.

The videos are each about 10 to 20 minutes long, and the parts of friends talking about Christensen's penalty have been deleted, prosecutors said.

According to The News-Gazette, the defense said that it had received the videos only recently, and not the edited versions that will be played.

The judge also allowed the prosecution to play clips of calls made by Christensen from jail.

Prosecutors said he asserted his innocence, claiming that the government didn't find anything, and that the case was political. The News-Gazette reported that prosecutors want to use the clips to show Christensen's lack of remorse.

There was frequent disagreement on preliminary jury instructions between the defense and prosecution.

The defense argued for Christensen to get life if only one juror doesn't agree with the death penalty, saying that many courts allow that to happen.

Otherwise, jurors, concerned with having a hung jury, could change their minds for expediency, the defense argued.

The judge said that for the preliminary instructions, the jury should try to reach a unanimous verdict, and he would decide about the final jury instructions later.

The court also went through a list of 54 mitigating factors the defense provided as to why Christensen shouldn't be sentenced to death. Those ranged from an alcoholic mother to Christensen not having a criminal history, to his allegedly not receiving proper care at the University of Illinois Counseling Center.

The judge allowed the factors to be presented, but warned the defense to be careful about connecting a previous diagnosis to Christensen's mental health on June 9, 2017, the day he kidnapped Zhang in his car and later killed her.

On June 24, the 12-member jury unanimously ruled that Christensen was guilty of all three counts against him, namely kidnapping resulting in the death of Zhang, and two counts of making false statements to the FBI.