Authorities believe Zhang, a University of Illinois student and a scholar from China, is dead. Brendt Christensen, 28, is charged in connection with her disappearance.
The criminal complaint against Christensen says his phone was used in April to visit FetLife.com, on which one thread was entitled, "planning a kidnapping." Christensen attended a vigil for her and prosecutors say he talked about the "ideal abduction victim."
Multiple people have reported seeing Zhang in Salem, Illinois.
Salem police say detectives have looked into the reports, but have not found concrete leads. The FBI says they are taking all tips and following through on them.
Despite investigators' belief that Zhang is dead, her family is still holding out hope that she is alive.
A business in Salem says her family has been in town, going off tips of sightings.
A US$50,000 (HK$390,000) reward is being offered for anyone that has information on Zhang's whereabouts.
Tom Bruno, Christensen's defense attorney, says his client deserves the "presumption of innocence."
Christensen is now in jail.
Bruno said that he doesn't think "a negative inference" can be made from prosecutors' statements that Christensen marched in a vigil for Zhang.