The parents of an American software engineer who believe their son was murdered last year in Singapore withdrew from the inquest Wednesday, saying they have no confidence in the city-state's legal process.

Shane Truman Todd's body was found in his Singapore apartment by his girlfriend last June 24, and police have said he killed himself. State counsel presented evidence of links to suicide websites on the 31-year-old's laptop and suicide letters written to his family members and loved ones.

But parents Rick and Mary Todd have said they consider the evidence fake. They told The Associated Press in March that they believe he may have been murdered over his research in the U.S. into material used to make heat-resistant semiconductors, a technology with both civilian and military applications. The Todds have received assistance in the case from U.S. senators and the FBI.

On Tuesday, a key witness in the ongoing inquest withdrew his initial claims that Todd was strangled. The state, meanwhile, introduced a witness who contradicts the parents' claim that their son was killed before June 23.

The parents, who traveled to Singapore from their home in Montana, issued a statement Wednesday saying they will no longer participate in the inquest and will return to the U.S.

"We have been told that the coroner's inquiry is not adversarial, rather it is a fact-finding mission with the sole purpose of getting to the truth," the statement said. "This has not been our experience. We no longer have confidence in the transparency in the findings of the system. It appears to us that the outcome has already been predetermined."

The parents, who had been expected to testify at the inquest, also have said the crime scene at their son's apartment was compromised, with key computer evidence mishandled. The FBI, however, has supported Singapore police claims that a hard drive found at Todd's apartment was handled by Singapore authorities who were checking for evidence, and not by an unknown person.

"We were looking forward to an honest and transparent court proceeding," the parents' statement said. "Sadly, this has not been the case. Therefore, we have decided that our presence in Singapore will have no bearing on the outcome of this case."

Singapore Foreign Affairs Minister K. Shanmugam said the government regrets the family's decision to leave the inquiry.

"For reasons best known to them, they walked out and it's unfortunate that they decided to leave," Shanmugam told a news conference.

"The family had said previously that they had found the hard drive themselves, and they asserted that this hard drive had been processed by a third party," he said. "It would have been useful to hear the family's side as to how they had come to a different view of the facts."

Gloria James-Civetta, a lawyer for the Todds, said the family would leave Singapore this week, but she would not comment on what their intentions were.

While Singapore authorities insist there was no evidence of foul play in Todd's death, the parents maintain he was murdered over his work researching semiconductor technology in a secretive project involving the Institute of Microelectronics and Chinese telecom giant Huawei.

Edward Harris Adelstein, a medical examiner based in Missouri who was engaged by the Todd family, withdrew key evidence Tuesday that Todd may have been strangled with a cord, saying he may have been killed by a taser followed by a deadly headlock. Adelstein said he had received new information from the family, including more autopsy photographs.

Testifying through a two-hour video call to the court, Adelstein engaged in heated debate with government lawyers over his speculation that "well-trained assassins" may have been involved in Todd's death. But when questioned by Deputy Senior State Counsel Isaac Tan, Adelstein said he had "no proof" for his claims on how Todd may have been killed.

Also on Tuesday, the court announced the introduction of a witness, Luis Alejandro Andia Montes, Todd's former colleague from the Institute of Microelectronics. Montes testified that he met Todd on the evening of June 23, the day before he was found dead, contradicting the parents' theory that their son was killed before June 23.

"He is claiming that my son was alive on the 23rd and we don't believe that," an emotional Mary Todd told reporters Tuesday.

District Judge Chay Yuen Fatt said the inquest would continue with the remaining witnesses. The inquest's conclusions are final and cannot be appealed.