The wife of the pilot at the helm of Malaysia Airlines flight MH370 told investigators that he stopped speaking to her in the weeks leading up to the ill-fated flight.

Madam Faizah Khanum Mustafa Khan had allegedly informed investigators that "he retreated into a shell", spending time alone in his room in the house they shared, Britain's The Mail on Sunday reported.

More than three weeks since flight MH370 disappeared with 239 crew and passengers aboard, investigators in Malaysia suspect that the plane may have been deliberately steered off course.

The plane is thought to have flown hundreds of miles out over the southern Indian Ocean where it eventually ran out of fuel and plunged into the sea.

Speaking about the mystery for the first time, the wife and daughter of Captain Zaharie Ahmad Shah said the 53-year-old pilot had been distracted and withdrawn in the weeks before the aircraft's disappearance.

According to the same report, no suicide note has been found and no motive established, but police are continuing to concentrate their inquiries on the pilot's background and whether his state of mind before the flight may be a factor.

Quoting a source close to the pilot's family, The Mail on Sunday reported that Madam Faizah told investigators that Mr Zaharie had stopped speaking to her and spent time alone in his room where he had built a flight simulator.

"He just retreated into a shell. I found him distant and difficult to understand," she said, adding that her husband was so withdrawn he hardly spoke to his sons and was not close to them.

Of his three children, Mr Zaharie appears to have been closest to his daughter Aishah, who flew back from Melbourne, Australia, to be with her family after the plane went missing.

She had told investigators that her last conversations with her father was a lot different to those they shared before.

"He wasn't the father I knew. He seemed disturbed and lost in a world of his own," she told investigators, according to the Mail on Sunday.

"He wasn't his usual self. He was distant and cranky," she had said, adding that she did not know if there was another woman in her father's life.

Among other revelations from the unnamed source were that Mr Zaharie was on the brink of divorcing his wife after nearly 30 years of marriage, but still refused to attend marriage counselling with Islamic elders.

Madam Faizah broke down repeatedly during two lengthy interviews with police, the family source told The Mail on Sunday, adding that one of the interviews lasted more than four hours.

However, despite the pilot's behaviour, his family are convinced that he was not responsible for the plane's disappearance.

According to the report, Madam Faizah refused to accept Mr Zaharie might be involved in the flight's disappearance, protesting: "It's unfair to blame my husband."

His daughter Aishah also told investigators she did not believe he could be in any way responsible for the flight's disappearance.

"I don't believe he would ever intentionally endanger the lives of his crew and passengers," she insisted.

According to the Mail on Sunday, police interviews with family members have confirmed that the pilot, who lived with his family in an upmarket suburb in Kuala Lumpur, did not have any obvious financial problems.