The son and daughter-in-law of Taiwan's ex-president Chen Shui-bian Monday returned to the island to face a probe into alleged money laundering implicating the former leader.

The couple, listed as defendants along with the ex-leader and his wife Wu Shu-chen over their alleged roles in the scandal, insisted they knew nothing about a 20 million US dollar fund kept at their bank accounts set up by a Swiss bank in Cayman Islands.

"I have no idea about the funds because my mother has long been the one handling the family's money," said the son Chen Chih-chung at the airport.

He stressed he did not know about the source and how the fund was wired abroad.

His wife, Huang Jui-ching, also said her mother-in-law, Wu Shu-chen, asked her to sign some documents, which she did as she was told without asking why.

They insisted that their return to Taiwan is a solid proof that they want to cooperate with the judicial authorities in clearing themselves and their family. Taiwan prosecutors have issued summons for the couple, who left Taiwan on August 9, just days before the scandal came to light earlier in August.

Taiwan's prosecution authorities have listed the former president Chen and four of his family members as defendants over their alleged roles in a high-profile money laundering scandal.

The scandal came to light on August 11 after Swiss judicial authorities notified Taiwan after finding unusual fund transfers through the son and daughter-in-law accounts set up by a Swiss bank.

The Supreme Prosecutor's Office launched the probe after Chen admitted at a news conference on August 14 that his wife had remitted 20 million US dollars worth of previous campaign donations abroad without telling him. He said his wife told him this was done in preparation for their retirement after he stepped down as president in May.

On August 16, prosecutors barred the ex-leader and his wife from going abroad, after searching Chen's home and office and Wu's home earlier in the day.

Investigators claimed to have found details of four secret bank accounts opened in 2000, the year Chen won the presidential election.

Chen said the money was from campaign funds that he had failed to declare. But prosecutors are trying to determine if the couple was attempting to launder illegally-obtained money through their son and daughter-in-law as well as other relatives.

The money laundering scandal has dealt a crushing blow to Chen's Democratic Progressive Party (DPP), which lost its eight-year grip on power to the pro-China Kuomintang (KMT) party in the March 22 election.

Chen, who served two four-year terms as president between 2000 and 2008, resigned from the DPP on August 15 as the party was preparing to expel him.