The "baby factory" case: The single billionaire who suddenly became a dad to 19 babies.

Mitsutoki Shigeta is 28. He is not married. He is extremely wealthy. He is believed to have 19 children, most of them around four years old.

And everyone has questions.

In 2014, the young Japanese man was at the centre of an international furore when it was revealed he had fathered a large number of babies through surrogate mothers in Thailand. Police raided his apartment in Bangkok and found nine babies, each one with a nanny. There were bottles and playpens everywhere. Interpol investigated, but found no evidence of human trafficking.

The “baby factory” case, as it was known, along with the “baby Gammy” case, led to the Thai government cracking down on commercial surrogacy.

Shigeta has already been awarded custody of four of his children born in Thailand, according to a CNN report.

This week, a Bangkok court granted him “sole parent” rights to the other 13. CNN reports his lawyer saying that he has another two children born via surrogates in India.

Shigeta is believed to be the son of a Japanese technology billionaire. When the court gave him custody of the 13 children, it was due to his “professional stability” and having an “ample income to raise all the children”.

The court also felt that growing up with a biological parent would be in the children’s best interests, ABC reports.

Through the court, it was revealed that Shigeta has already been making plans for his children’s future in Japan. He has bought land near a big park and intends to build a house there, and to hire carers and professional nurses to look after them.

Until now, the 13 children have been living in government care in Thailand. Shigeta has been paying for them to have English and Japanese tuition. His mother has been travelling from Japan to visit them every month.

So why did Shigeta decide to father so many children at the same time? He's always maintained that he simply wants a "big family".

"He has personal and business reasons," his lawyer, Kong Suriyamonthon, explained this week. "He was born in a big family, so he wants his children to grow up together."

But who are the mothers? No information has ever been revealed about where the donor eggs came from. In 2014, the Bangkok Post reported a police doctor saying that it appeared the children were conceived from eggs from "various races" of women. The children were described as looking "very different".

As for the surrogate mothers, one of them, who only wanted to be known as Wassana, told her story to the Japan Times in 2014.

LISTEN: Surrogate Shannon Garner speaks about what it’s like holding a baby you can never keep. Post continues after audio.

She was living in a slum and her family were in debt due to her late father's medical bills when she saw an ad offering $10,000 for a woman to carry a child for parents who couldn't conceive. Without asking too many questions, Wassana became a surrogate. She developed pre-eclampsia and gave birth to a boy two months early via caesarean. She said that Shigeta didn't visit her in the hospital, and that she met him for the first time two months after the birth, when she had to sign a document granting him sole custody.

"He didn't say anything to me," she said.

She said she only held the baby once, briefly, in court.

The money Wassana received paid off her family's debts, but it wasn't enough for them to move out of the slum.

It was a year before Wassana saw Shigeta on TV and found out the full story. At the time, she told police she would be willing to raise the baby if he was being mistreated.

"I thought he would be with a good family that would love him," she said.

One thing's for sure - all the children will be well looked after financially, with Shigeta already setting up trust funds for them.

As for Shigeta, his lawyer reports that he's "very, very happy".

Watch Shannon Garner discuss her experience of being a surrogate in the video below.