An American engineer who was found dead in his apartment in Singapore last year killed himself, a judge has ruled.

Shane Todd, 31, had been working for Singapore Institute of Microelectronics (IME) but left his job at the state-owned research organisation just two days before his death. He was found dead at his home by his girlfriend in June 2012.

Todd's parents alleged he may have been murdered over his research in the US into material used to make heat-resistant semiconductors. It's a technology with both civilian and military applications, products like LED screens, military radar and satellite systems.

They also said they found a hard drive in their son's apartment that contained documents he had backed up from his work computer, including a draft of a project outline between IME and Chinese telecom giant Huawei on this technology. IME has said neither Todd nor the company was involved in any classified research, while Huawei has said it had no cooperation with the institute related to that material, gallium nitride.

Huawei has been blocked from some projects in Australia and is deemed a security risk by the US Congress on the grounds that its equipment could be used for spying.

State counsel presented evidence of links to suicide websites on the 31-year-old's laptop and suicide letters written to family members and loved ones. Judge ChayYuen Fatt said on Monday he found no evidence of foul play, ruling that Todd committed suicide by hanging himself.

Chay recorded the official cause of death as "asphyxia due to hanging". He also added: "The deceased was not in possession of confidential and valuable classified information in the course of his employment at the IME." The inquest ruling cannot be appealed.

The case had threatened to become an issue between Singapore and the US as Senator Max Baucus, who represents Todd's home state of Montana, had pressed for more American involvement in the investigation.

K. Shanmugam, Singapore's minister for law and foreign affairs, said the city-state had cooperated with U.S. authorities, including the FBI, and acted according to the law to get to the truth.

The U.S. Embassy in Singapore said in a statement that the inquiry had been comprehensive, fair and transparent.