It is "regrettable" that the family of American researcher Shane Todd has decided to not to participate at the inquiry into his death, said Law and Foreign Minister K Shanmugam on Wednesday in a press conference.

This is because the family could have answered questions about their assertions that have been contradicted by the State Counsel's investigations, he said.

For instance, the Todds have claimed they found a hard drive in Dr Todd's apartment containing sensitive documents about his work at the Singapore Institute of Microelectronics (IME).

However, the police have said that they were the ones who had taken the hard drive and reviewed it before handing it to the family. This is also supported by the United States Federal Bureau of Investigations (FBI).

The Todds also claimed that they did not know a witness who testified that he had last seen Dr Todd alive the night before his body was discovered on June 24 last year. On Wednesday, this witness, Dr Luis Montes - who was a colleague of Dr Todd - said he had actually met the family when they came to Singapore last year.

"These would have been better cleared if the family chose to show up," Mr Shanmugam said, who also noted that the family had expressed faith in the judiciary just last week, but walked out of the proceedings on Wednesday a day after their medical expert retracted part of his findings and admitted he was not a certified forensic pathologist.

"We hope they will take part in the inquiry. If they don't, it is regrettable but the inquiry will continue," he said.

Asked about a statement put out by the Todds on Wednesday which cast aspersions on Singapore's judiciary system, and their allegations about improper police work, Mr Shanmugam said: "The police witnesses have gone on the stand... It is for the world to judge for itself."