Shane Todd, 31, was employed as an engineer for a technology firm in Singapore when he was found dead in his apartment in June.
"I know he was murdered and then they just hung him on a door," his mother Mary Todd, told ABC News after she and her husband Rick Todd returned from Singapore to examine evidence.
The bathroom where Shane Todd was found, with allegedly an elaborate system of pulleys and ropes, did not match police descriptions, the Todds said.
"I saw no screws in the walls, no ropes, no pulley. I called the police immediately and said the description you gave me does not line up with the physical evidence," Rick Todd said.
An alleged suicide note, found on a computer in the apartment, was impersonal and not in the dead man's tone, Shane's brother Dylan said.
An American pathologist who examined Shane Todd's body, at his parents' request, said bruise marks indicated he died after a struggle, Rick Todd said.
The parents said a backup hard drive in their son's apartment had evidence suggesting he might have been caught in a plot to transfer sensitive technology to China.
The Todds enlisted the aid of Montana's two U.S. senators, Max Baucus and John Tester, who have pushed to have U.S. investigators take the lead in reviewing the case, ABC said.
K. Shanmugam, Singapore's minister of foreign affairs, said after meeting with U.S. Secretary of State John Kerry March 12 Singapore police agreed to share withheld evidence with the FBI.