In May 2017, Zhang during field research and equipment setting trip to Nebraska. [Photo provided to China Daily]

"Life is too short to be ordinary," wrote Zhang in her final diary entry on June 1, eight days before her death.

On June 29, 2017, that sentence was cited by Hou at a concert held in Zhang's name at UIUC, where the 26-year-old had been doing field research on crop photosynthesis.

Describing her venturing-out amidst a downpour during her brief stay at UIUC, another, earlier entry is drenched in loneliness. "My umbrella couldn't keep out the rain and my glasses were covered with raindrops," she wrote. "As cars were passing, I was thinking: it must be very warm inside there..."

On June 12, 2017, three days after Zhang was reported missing by Miao Guofang and Liyan, two of her fellow researchers in UIUC's Department of Natural Resources and Environmental Studies, police located security camera footage from a parking garage that showed her getting into a black Saturn Astra on the university campus.

The day before hope was lost - people in the university town turned out for Zhang on June 29, 2017. [Photo provided to China Daily]

"Now we know that Christensen, who had failed in his attempt to lure another young lady into his car that same morning, was disguising himself as a cop," Shi says. "Moreover, Yingying was already running late for an appointment she made with a rental manager. She didn't want to be late."

Shi and Sun met on a mobile chat group formed as a direct result of Zhang's going missing in mid-June, by mostly Chinese students in the Chicago-Champaign area who wanted to help. Sun, a physics student who had just discovered his interest in cinematography at the end of a four-year bachelor's program in UIUC, was instantly drawn to Shi's idea of filming the fast-unraveling event.

Christensen (left center, shown in silhouette) and his then girl friend Bullis (center with braided hair) in the march for Zhang on June 20, 2017. [Photo provided to China Daily]

The concert was the first event Sun had filmed. Little did he know at the time that among those whom his camera had captured was Christensen and his girlfriend Terra Bullis, who by that time had been wearing a wire for the FBI for nearly two weeks. Christensen had just graduated with a master's degree from UIUC's physics department, the same one as Sun had been attending.

As Hou brought an aching silence to the venue with a guitar rendition of a song he wrote for Zhang, Christensen took Bullis' mobile and typed the words "It was me", followed by "she is gone ...FOREVER".

In graphic detail Christensen described to Bullis how he tormented and sexually assaulted her in his bedroom, before carrying her into a bathtub, where he hit her "as hard as he could with a baseball bat". Despite his seemingly uncontrollable urge to brag about the atrocity, Christensen expressed disbelief at the valiant defense of his victim, who even "reached up to grab" his hand as he stabbed her neck.

"She just didn't give up," said the killer, who eventually decapitated her.