It also included the man suspected of kidnapping her, officials say.
University police confirmed the man standing by himself in a dark T-shirt with his left hand on a railing as Christensen, said Robin Kaler, associate chancellor for public affairs at the University of Illinois.
The day after the rally, Christensen was arrested on suspicion of kidnapping. And while many were surprised Christensen would show up to the rally, the suspect had actually been under surveillance for weeks.
Suspect's phone visited 'Abduction 101' forum
A spate of bizarre circumstances led to Christensen's arrest. Among those cited by FBI Special Agent Anthony Manganaro in a criminal complaint
-- Christensen's unusual car, a black Saturn Astra, had a sunroof and a cracked front passenger hubcap, just like the vehicle that picked up Zhang the day she disappeared.
-- The suspect changed his story about what he was doing the afternoon of June 9, the day of the disappearance, Manganaro wrote.
Christensen said he was probably sleeping or playing video games "at his residence all day," the FBI agent wrote. A few days later, Christensen told investigators he was "driving around the UI campus when he observed an Asian female with a backpack standing at a corner appearing distressed," Manganaro wrote.
Christensen told authorities he dropped the woman off a few blocks away, the complaint said.
-- An investigation into Christensen's phone revealed a visit to the fetish website FetLife, where the user visited a forum called "Abduction 101,
" Manganaro wrote. Threads on that forum included "Perfect abduction fantasy" and "planning a kidnapping," the complaint said.
-- Investigators sought search warrants and started "continuous surveillance" on Christensen around June 16. On June 29, the day before he was arrested, Christensen "was captured on audio recording ... explaining how he kidnapped Y.Z.," or Yingying Zhang, the FBI agent wrote.
"Christensen stated that he brought Y.Z. back to his apartment, and otherwise held her in his apartment against her will," Manganaro said.
The agent said Zhang is still missing and presumed dead.
'My scalp was tingling'
Yuan Ziwen, a roommate and close friend of Zhang's boyfriend, said he was frightened when he saw the video released by police.
"My scalp was tingling. Yingying is a really cautious and reasonable person, I don't believe she will enter some stranger's car unless there is an emergency or she is in a rush," Yuan told CNN.
As Chinese news outlets continue to provide extensive coverage on the Zhang case, public interest in the story shows no sign of abating.
While sympathy for the victim appears universal, heated debates have also broken out on social media platforms on topics ranging from personal safety in America to the efficiency of US law enforcement agencies.
A commentary in the Global Times, a provocative but state-sanctioned tabloid that often takes hard-line positions, concluded the FBI mishandled the investigation because "police were overwhelmed" by the high crime rate in the United States.
"In fact, the FBI's inefficiency is not by accident, yet difficult to avoid; this is tied to the high crime rate and judicial system in the US, also, the problems of FBI itself," the paper said.
Many current and former US-based Chinese students and scholars have also brushed aside the notion that the Zhang case highlights the danger of living in America -- in sharp contrast to major Chinese cities where violent crimes are still relatively uncommon.
"I'm not afraid of going to the States at all," said Zhong Siqi, a 22-year-old recent college graduate in southern China who will attend Northeastern University in Boston in the fall.
Defense attorney: 'Keep an open mind'
Christensen appeared in a magistrate's court for an arraignment hearing Monday, but said very little. He did not enter a plea for the kidnapping charge.
Another hearing will take place Wednesday to determine whether the 28-year-old should remain in custody, said Sharon Paul, spokeswoman for the US Attorney's Office for the Central District of Illinois.
Christensen's defense attorney, Evan Bruno, urged the public to avoid jumping to conclusions.
"There's a long road ahead," Bruno said. "I encourage everyone to be patient, to keep an open mind and wait until the evidence comes in."
A love of watching fireflies at night
In an interview Tuesday with CNN, Guofang Miao, Zhang's colleague, said she was "sad and angry" when she saw the man in court charged with kidnapping her friend.
Miao, a post-doctoral student at the university, worked closely with Zhang for more than a month on a project with the university's College of Agriculture. They called themselves field ecologists and were concerned about the environment, said Miao, who holds a Ph.D. in forestry and environmental resources from North Carolina State University.
The pair used new technology to develop methods to predict crop yield and studied how new crops adapted to climate change, Miao said.
"The work is really tough but we enjoyed (the work) a lot," Miao said, standing in the university corn field where the two worked.
Zhang was talented and passionate about her work, Miao said. But she also recalled the lighter side of her friend.
They talked about the corn stalks that sprang up quickly and the beautiful fireflies at night that fascinated Zhang, said Miao, a native of China. Zhang didn't see fireflies where she studied in China.
"When she started to see those she was so excited," Miao said. "I said 'okay, if you really like, I can drop you here and you can stay until night. ... Then I can come to pick you up later.' "
"I still hope she can make it back to see those," Miao added. "She will love that."