A man charged with kidnapping a student at a bus stop had visited a forum called 'Abduction 101', the FBI said.
Brendt Christensen, 27, was arrested on Friday by cops investigating the disappearance of Yingying Zhang, a Chinese student who was visiting the University of Illinois.
She was last seen on June 9 when CCTV showed her being approached by a driver in a black Saturn Astra car as she waited at a bus stop.
After several minutes conversation, she got into the car and has not been seen since.
The FBI began investigating and spoke to Christensen, a fellow student who owns a black Saturn Astra.
He offered no information on the whereabouts of Yingying, but according to court documents, later boasted of having kidnapped her while under surveillance.
Cops then searched the 27-year-old's phone and found he was part of the social network FetLife which caters for the "the BDSM, Fetish & Kinky Community."
He found the 'Abduction 101' forum via FetLife and visited sub-threads called 'Perfect abduction fantasy' and 'planning a kidnapping', the FBI said.
Prosecutors said based on Christensen's kidnapping boast "and other facts uncovered during the investigation" they believe Yinying "is no longer alive".
University of Illinois Chancellor Robert Jones wrote to staff of his dismay at the events.
Yinying, who was studying studying photosynthesis and crop productivity, had only arrived at the university in April.
He said: "There are no words that can explain why or how such a terrible thing should happen nor is there anything I might say that will ease the grief of any of you who knew her.
"I ask all of you to help to ensure that Yingying is remembered for her kindness, her gentleness and her smile."