But that's not the only reason police are accusing him of abducting the Chinese grad student at the University of Illinois.
A spate of unusual circumstances led authorities to charge Christensen with kidnapping. But while police now have their suspect, the victim is still nowhere to be found.
Here's what we know -- and what the FBI says -- about the day Zhang disappeared and about a case that has spawned concern across two continents:
Her family had suspected that Zhang was running late to an appointment to sign an apartment lease, and that may have led to her disappearance.
"We can't think of any other reason that she would randomly jump into a stranger's car," Zhang's aunt, Liqin Ye, told CNN
In fact, at 1:39 p.m. on June 9, Zhang texted the manager of the apartment complex to say she was going to be late, FBI Special Agent Anthony Manganaro wrote in a criminal complaint
After that text, surveillance video showed Zhang trying to flag down a bus, but to no avail. She then walked to and waited at a bus stop. A few minutes later, a black Saturn Astra drove past her.
The suspect's car "appears to have passed (Zhang) at the bus stop and circled the block back to her location," Manganaro wrote.
The driver stopped to talk to Zhang, and the two spoke for about a minute, the FBI agent said in the complaint.
The 26-year-old grad student then got into the car -- and disappeared.
Complaint: The car's rarity helped lead to the suspect
From the surveillance video, the driver's identity was hard to tell. But he drove an unusual car. And there were only 18 four-door Saturn Astras registered in Champaign County, Illinois, the FBI agent wrote.
One of those Astras was registered to Christensen. And when investigators visited Christensen's apartment complex, they discovered his car had a sunroof and a cracked front passenger hubcap -- just like the car in the surveillance footage.
Authorities got a warrant to search Christensen's vehicle and noticed the front passenger door of Christensen's car "appeared to have been cleaned to a more diligent extent than the other vehicle doors," the criminal complaint says.
"I believe that this type of action may be indicative of an attempt to conceal or destroy evidence," Manganaro wrote.
Christensen's first court appearance is scheduled for 10 a.m. Monday at a federal court in Urbana. It's not clear whether the suspect, who is still in custody and has not yet entered a plea, has an attorney.
FBI: Christensen changed his story
When authorities interviewed Christensen on June 12, he initially said he couldn't remember where he was between 2 p.m. and 3 p.m. the day of the kidnapping, the FBI agent wrote.
"He later advised, however, he must have been either sleeping or he was playing video games at his residence all day on June 9," Manganaro wrote.
But when authorities interviewed Christensen again on June 15, "Christensen admitted to driving around the UI campus when he observed an Asian female with a backpack standing at a corner appearing distressed," the FBI agent wrote.
"Christensen claimed that he drove up to the Asian female, who relayed she was late to an appointment. Christensen stated that he offered the Asian female a ride."
According to the criminal complaint
, Christensen said he may have made a wrong turn, and the woman panicked. At that point, he said, he let his passenger out of the car a few blocks away from where he had picked her up.
'Abduction 101' forum visited, complaint says
Investigators searched Christensen's phone and discovered a visit to the 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.
Efforts by CNN to reach someone at the website were not successful on Sunday.
Agent: Suspect caught talking about kidnapping
Investigators obtained search warrants and started "continuous surveillance" on Christensen. And authorities say the ensuing evidence contradicts Christensen's claim that he had let Zhang get out of his car a few blocks away from where she was picked up.
"Christensen was captured on audio recording while under law enforcement surveillance explaining how he kidnapped Y.Z.," or Yingying Zhang, the complaint said.
"Christensen stated that he brought Y.Z. back to his apartment, and otherwise held her in his apartment against her will," the complaint said. It did not elaborate on the circumstances of the recorded remarks.
Based on that finding and others, Manganaro wrote, "law enforcement does not believe Y.Z. is still alive."
Victim and suspect attended the same university
Christensen was a graduate student in the physics department and a teaching assistant until May, a University of Illinois professor told CNN.
"Everybody is shocked by this," said Lance Cooper, an associate head for graduate studies in the department.
"There were no flags, no discipline issues with him, no problem with his teaching, no problems with his coursework."
Cooper said he didn't think Christensen would have had any academic interactions with Zhang, who came to the United States within the past two months.
David Meldgin, a teaching assistant in the physics department, said he had an office next door to Christensen for two years, though they spoke only a few times.
"He was a quiet person in meetings, but he always helped students who needed help." Meldgin said. "I'm really surprised."
Zhang's disappearance triggered widespread concern in China, especially on social media. There are more than 300,000 Chinese students and scholars attending US universities.
Zhang had a yearlong position at the university's department of natural resources and environmental sciences. She graduated from Beijing's prestigious Peking University last year with a master's degree in environmental engineering and wanted to learn more in the United States.
"Her mom ... being a mom, (was) obviously reluctant to let her go afar," Zhang's aunt Liqin Ye said. "And her mom's thought is kind of a traditional way. ... 'This is the age for you to get married, have a family. There's no need to go this far, to the United States.' "
But Zhang was determined to chase her dream of studying environmental sciences and becoming a university professor, her aunt said.
"To be a faculty member in China, there are certain qualifications you have to meet, and as part of that you have to have broad experience in terms of your research and your study," Ye said. "And that's probably what motivated her to want to come to the United States."