A US high-tech researcher whose family claims he was murdered in Singapore was under treatment for depression and left suicide notes before he was found hanged, a public inquiry was told on Monday.

Senior state counsel Tai Wei Shyong, opening a coroner's inquest into the death of electronics engineer Shane Todd, said there were no signs of foul play in the apartment where his body was found.

Todd's parents, who are taking part in the inquiry, believe he was killed in connection with his work for a Singapore research institute with alleged links to a Chinese firm accused of involvement in international espionage.

Huawei Technologies, a Chinese telecom giant, and Todd's former employer, the state-linked Institute of Microelectronics (IME), have denied collaborating on any project involving Todd, who was 31 when he died.

Todd's body was found by his girlfriend, Filipino nurse Shirley Sarmiento, at his apartment in the evening of June 24, 2012, setting off a saga that has reached the highest levels of the US and Singapore governments.

Tai said police found Todd's body hanging from an improvised noose attached to the top of a bathroom door. "They did not observe any signs of foul play," he said.

Police found a handwritten note with a password to Todd's laptop computer, which contained messages to his family, friends and girlfriend, Tai said.

The state counsel reproduced one note addressed to "Mom and Dad" saying that "I am facing problems that I don't know how to solve."

The note added: "I hope that you understand that I am sorry for the pain this causes. I just know how much of a burden I will be to you in the future so I feel it is better to do this now rather than wait until I have caused more damage."

Todd's parents Rick and Mary listened calmly in the packed courtroom.

Sarmiento said in a statement to police that Todd was "behaving coldly" in their last weeks together, often complaining about his job, but added that "he did not display any suicidal tendencies".

Todd's neighbour Michael Goodwin agreed, testifying that "he did not seem like a person at all who would take his own life".

The inquiry, with over 60 potential witnesses, is limited to determining the cause of death. A verdict is expected by late June.

Tai said that three months before his death, Todd sought medical help and was prescribed anti-depressant pills by a Singaporean psychiatrist after stating in a patient questionnaire that he was struggling with "stress, anxiety and depression due to work and life-related issues".

Further forensic tests of Todd's laptop revealed an Internet search history showing repeated visits to suicide-related websites, Tai said.

Todd used search keywords like "noose", "hangman", "short drop" and "simple suspension" between March 10 and June 23, 2012 -- the day before his body was found.

Todd arrived in Singapore in November 2010 to work for the IME and resigned in May 2012. His last day of work was June 22 and he was preparing to return to the United States when he was found dead.

Todd's family said in media interviews that their son in telephone calls expressed fears for his life because he was afraid he was being asked to compromise US security in his job.

After strong lobbying by the Todd family in Washington, the Singapore police invited the US Federal Bureau of Investigation to help probe his death.

Huawei and the IME have both said they held preliminary talks on collaboration but did not proceed with any joint project.

Todd was part of an IME team working on gallium nitride, a tough semiconductor material that can be used in radar and satellite communications.

A US congressional committee last year labelled Huawei and ZTE, another Chinese telecom firm, as potential security threats that should be excluded from US government contracts and barred from acquiring US firms.