Edward Adelstein, 75, a deputy medical examiner in Missouri, contradicted Singapore police findings that Shane Todd killed himself, but admitted his conclusions were based on pictures of the body and circumstantial information.
Todd's parents, who also plan to testify, say their son was killed in June 2012 because of his work for a Singapore electronics research institute with alleged links to a Chinese firm accused of involvement in espionage.
He said Todd was "a very dangerous person" to the two Asian companies, and asserted without offering any evidence that "they had him killed" and well-trained "assassins" may have been involved.
Adelstein was engaged by the Todd family. Two other US medical examiners acting as independent experts support the suicide findings and will be asked to testify at the two-week inquest, Singapore officials say.
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Under questioning during a live video link from the United States, Adelstein said Todd could have been disabled with a taser -- an electronic device designed to stun -- and killed with an arm lock before being hanged.
In an October 2012 report, Adelstein had said that Todd, a well-built 31-year-old, was killed by "garroting" but on today the doctor said he was speculating at the time.
Todd's former employer, the Institute of Microelectronics (IME), and China's Huawei Technologies have denied working on a project involving Todd but confirmed they held preliminary talks on a potential research venture.