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Experts: Engineer found dead didn't write suicide notes

Was death of an American computer engineer in Singapore a suicide -- or, as his parents contend, murder? The mystery has possible links to the dark world of cyber-espionage.

Ever since the body of American engineer was found hanging in his Singapore apartment in June 2012, his mother Mary Todd has maintained that the five typewritten suicide notes found at the scene were not written by her son.

"My son did not write those suicide notes," Mary Todd has repeated to anyone who would listen.

Watch: "48 Hours:" Spies, Lies and Secrets

A Singapore coroner concluded after a lengthy inquest and investigation that Shane had written those notes and hanged himself because he was depressed. Mary Todd and her family have rejected that conclusion and continue to maintain that Shane was murdered because he would not agree to transfer high-technology secrets to a Chinese company.

In a new book on the case - Hard Drive: A Family's Fight Against Three Countries - Mary Todd and co-author Christina Villegas provide the findings of two experts whose conclusions are exactly what Mary has been claiming - that Shane did not write those notes. The experts made their separate determinations after the inquest was concluded.

Mary Todd spoke about the book on "CBS This Morning" last week, and a "48 Hours" investigation into Shane's death aired Saturday.

Dr. Carole Chaski, the executive director of the Institute for Linguistic Evidence in Delaware, sent the Todds an "official assessment" -- according to the book -- and that assessment states: "The alleged suicide note is not a real suicide note."

Dr. Chaski made that determination after running Shane's notes through a piece of software that analyzes text and said she would be willing to testify to that in court of law, the book states.

The notes also were examined by a second expert Dr. David Camp, a professor of criminal justice at Blackburn College in Illinois, and, according to the book, he wrote a ten page analysis that concluded the notes were "not written by a person who was socialized in the same culture or with the same linguistic pattern" as Shane Todd.

Over the past summer, two pieces of evidence - the noose and towel found around Shane's neck -- were destroyed by the Singapore police, according to an official letter sent to the Todd family lawyer earlier this month.

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Shane's parents Rick and Mary Todd of Montana, had been asking for months that the Singapore government return the items that were found around Shane's neck when his body was discovered hanging on the bathroom door of his apartment.

Both the noose and the towel contained the DNA of two unknown persons, according to reports done by Singapore authorities. No further testing was done by authorities to determine whose DNA it was.

"Rick and I wanted the towel and the strap returned to us in order to do what the SPF should have done in the first place -- to test the Chinese and Malay DNA (present on the items)," Mary Todd told CBS News. "We have ample evidence that our son was murdered, but the towel and the strap were the only DNA evidence in Shane's case, and now they have been destroyed forever."

Mary Todd also pointed out that the items belonged to Shane.

The Singapore Attorney General countered in a report that it is "normal protocol" for items used in hanging cases to be "forfeited to the state for disposal."

The Todds had retained a Singapore lawyer and the issue of what would happen to the noose and towel had become the subject of emails, official letters and hearings in recent months.

The Singapore government wanted the family to explain why they wanted the items returned but their Singaporean lawyer refused the request. "In law, there is no reason to explain why you want your property back," he wrote to the family. "It's like justifying to a robber why you want your belongings returned. Sheer chutzpah. It's the family's by right."

In July, 2013, the state coroner in Singapore concluded that Shane's death was caused by "asphyxia due to hanging" and ruled no foul play was involved. The Todd family has never accepted the official findings and, to this day, believes their son was murdered and that the crime scene was staged to make it appear he had taken his own life.

The Todds insist Shane was murdered after he was asked to compromise U.S. security by revealing secrets to a Chinese company. That company -- Huawei Technologies -- has been declared a threat to U.S. national security by the House Intelligence Committee. At the time, Huawei was discussing a joint project with Shane's Singapore employer, the Institute of Microelectronics.

But both companies deny their joint business discussions involved any classified military information and pointed out that the project was abandoned soon after Shane's death.

Paul LaRosa is a "48 Hours" producer.