U.S. prosecutors will seek the death penalty for a former physics student charged with the kidnapping and killing of a University of Illinois scholar from China, they told a judge in a Friday filing that also made a new allegation that the 28-year-old suspect once choked and sexually assaulted someone else years ago.

The filing in U.S. District Court in central Illinois provides several reasons for why the death penalty is called for in Brendt Christensen's case, including because he allegedly tortured 26-year-old Yingying Zhang before killing her. It didn't say how.

The new allegation is that Christensen "choked and sexually assaulted" someone referred to only by the initials "M.D." in 2013 in central Illinois. He has not been charged in that alleged assault. Christensen also once expressed his aspiration "to be known as a killer," the filing says.

Zhang disappeared June 9 on her way to sign an apartment lease off campus in Urbana, some 140 miles (225 kilometers) southwest of Chicago. She had arrived on campus in April and had just missed a bus when Christensen lured her into his car, prosecutors say. They say Zhang is dead, though her body hasn't been found.

Christensen, who earned a master's degree in physics from the University of Illinois, has pleaded not guilty to kidnapping resulting in death. His trial is scheduled to begin on Feb. 27, though his attorneys have said previously they would need more time to prepare, especially if the government intended to seek the death penalty.

Illinois abolished the death penalty in 2011, years after then-Gov. George Ryan declared a moratorium on executions, citing doubts about the guilt of several of those on death row. While capital punishment is available under federal law, prosecutors have sought it infrequently and it's rarely been carried out.

There have been just 37 federal executions — from 1927 to 2003, when the last federal execution occurred, according to the U.S. Bureau of Prisons website. The last was of Louis Jones, executed by lethal injection in Terre Haute, Indiana, on May 18, 2003, for a kidnapping resulting in death. Federal authorities have only executed one person in Illinois, in 1938 by hanging, according to the bureau.

Among other factors Friday's five-page filing says justifies the death penalty was the "heinous, cruel, or depraved manner" of the crime and that it involved "planning and premeditation," as well as what the document says is Christensen's "lack of remorse."

"The victim was particularly vulnerable due to her small stature and limited ability to communicate in English," the filing says.

A message seeking comment from Christensen's attorney, Robert Tucker, wasn't immediately returned.

Zhang, who received her master's degree in environmental engineering in China in 2016, had hoped to eventually land a professorship and help her family in China out financially. Her father, a sometime-semitrailer driver, traveled from China to Illinois in June to help look for his daughter.

Her disappearance prompted a massive search that drew international media attention, particularly from China.

Other disturbing details in the case emerged after Christensen's arrest last year.

Months before Zhang went missing, his phone was used to visit the website FetLife.com, including to view threads titled "Perfect abduction fantasy" and "planning a kidnapping," prosecutors say.

FetLife described itself as "the Social Network for the BDSM, Fetish & Kinky Community," stressing in policy statements that it's a place for consenting adults. The acronym BDSM stands for bondage, dominance, sadism and masochism. Users provide their ages, genders and roles they wish to play, but otherwise remain anonymous.

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