SINGAPORE - Former China tour guide Yang Yin was called in by the police and Immigration Checkpoints Authority (ICA) for an interview on Tuesday.

An ICA spokesman on Wednesday confirmed that Yang is "assisting the police with investigations" at the Police Cantonment Complex.

The Straits Times understands that Mr Yang, 40, has also surrendered his passport to the police. On Wednesday morning, two unmarked police cars and several plainclothes officers were spotted outside the Gerald Crescent bungalow belonging to 87-year-old widow Madam Chung Khin Chun, whose $40 million assets is now the subject of a civil court tussle. The police officers spent more than an hour at the bungalow, interviewing Madam Chung's niece Madam Hedy Mok and a maid.

Mr Yang, now a Singapore permanent resident, met Madam Chung in 2008 when he was her private guide while she was on holiday in Beijing with her long-time friend Madam Chang Phie Chin. The wealthy widow, whose husband died in 2007, has no children and was diagnosed with dementia this year. She made a will in 2010 leaving all her assets to Mr Yang, after he moved into her bungalow in 2009. In 2012, she also appointed him her guardian, giving him full control of her assets worth $40 million.

Her niece, 60-year-old travel agency owner Madam Mok, started court proceedings to revoke Mr Yang's guardianship and recover control of her aunt's assets earlier this year. The pre-trial conference is expected to take place on Sept 24.

Madam Chung, whose husband died in 2007, has no children and was diagnosed with dementia this year.