A University of Illinois graduate student told jurors Tuesday that the man on trial for killing a 26-year-old visiting scholar from China tried to get her into his car by posing as an undercover police officer, saying she felt physically ill when she recognized him days later in a police photo display.

This 2016 selfie provided by her family shows Yingying Zhang in a cap and gown for her graduate degree in environmental engineering from Peking University Shenzhen Graduate School. The 26-year-old visiting scholar at the University of Illinois at Urbana-Champaign, disappeared June 9, 2017. Brendt Christensen, a former graduate student, has been charged with kidnapping and killing her. Zhang's body has not been found. [Photo: AP]

Emily Hogan testified that Brendt Christensen, now 29, pulled up to her as she walked alone to a bus stop on the morning of June 9, 2017, the day Yingying Zhang went missing. Christensen was arrested on June 30, later pleading not guilty to a federal charge of kidnapping resulting in death. If convicted, he could be sentenced to death.

Hogan said Christensen, a former doctoral student at the university, wore mirrored aviation sunglasses and flashed a fake badge after pulling alongside her. She said he told her: "I'm an undercover cop. Could I ask you some questions?" Champaign's News-Gazette reported from the trial in U.S. District Court in Peoria.

She initially approached the car but then backed away when Christensen asked if she would get in. As he drove off, she immediately called police to report the driver and his suspicious behavior. She also posted a comment on Facebook about what had happened, as a warning to others.

Parents Lifeng Ye (L), Ronggao Zhang (R) and brother Zhengyang Zhang (C) of Yingying Zhang arrive at the US Courthouse as jury selection in the federal trial of Brendt Christensen begins in the 2017 disappearance and suspected killing of Yingying Zhang, a visiting scholar from China whose body has not been found on June 3, 2019 in Peoria, Illinois. [Photo: AFP]

Christensen began driving around near campus starting that morning, hunting for a victim to fulfill a homicidal fantasy, prosecutors allege. Four hours after he failed to talk Hogan into his car, he pulled up to Zhang at a bus stop as she was heading to sign an apartment lease off campus. She got in.

Prosecutors say Christensen later forced her into his nearby apartment, where he raped and stabbed her, and beat her to death with a bat.

After police had already identified Christensen as a possible suspect in Zhang's disappearance, Hogan said she was "shocked” to recognize Christensen as the man impersonating an officer several days before. She said she felt overwhelmed.

"I was sick," she told jurors.

Prosecutors are expected to call their star witness, Christensen's former girlfriend, sometime this week. Soon after her testimony, they would rest their case. The defense, which already conceded in opening statements last week that Christensen killed Zhang, would then have their chance to address the jury.

Mother Lifeng Ye (R) and brother Zhengyang Zhang (C) of Yingying Zhang arrive at the US Courthouse as federal trial of Brendt Christensen begins in the 2017 disappearance and suspected killing of Yingying Zhang, a visiting scholar from China whose body has not been found on June 12, 2019 in Peoria, Illinois. Zhang Yingying went missing in June 2017 when police say she got into a car belonging to Brendt Christensen near the campus of the University of Illinois. She was 26 years old at the time. [Photo: AFP]