Ma had already been summoned five times since Aug. 27 for questioning as a witness in the probe into the case that Chen's family and several aides were alleged to use other people's invoices or receipts to claim the "state affairs fee" that can only be used by the president for public purposes.

For his sixth appearance at the SID under the Supreme Prosecutor Office, he was summoned with a new status of defendant.

Ma was questioned for almost seven hours in the company of a lawyer before prosecutors decided to place him into custody.

Like several of Chen's aides who were already held at a detention center, Ma also played a role in stashing away public funds, said prosecutors.

The prosecutors wanted to detain Ma on grounds that the criminal case in which he is allegedly involved potentially carries a jail term longer than five or even 10 years.

The court granted the permission to place Ma in custody after holding a hearing in the late evening.

Ma, 43, was a confidant of President Chen during the days when Chen served as a legislator and campaigned for Taipei mayor on behalf of the Democratic Progressive Party (DPP).

After Chen was elected as Taipei mayor, helped by the splitting of votes for two candidates in the Kuomintang camp, Ma successively held the positions as director of the Mayor's office, chief of the Department of Information, and deputy secretary general of the Taipei City Government.

After Chen won the presidential election in 2000 again due to the division of the KMT, Ma became director of the president's office and was later promoted to deputy presidential secretary general.

With special trust from Chen, Ma was also appointed as the chief executive officer of the Formosa Foundation, which was set up by Chen mainly with unused campaign funds.

The trial of the case involving the first family and officials at the Presidential Office has been repeatedly delayed by the absence of the wheelchair-bound former first lady Wu Shu-chen who cites doctors' diagnosis that she is not fit to appear in court due to health reasons.

When questioning Ma, prosecutors also summoned four witnesses, including Ma Jui-lin, a former chief comptroller at the Presidential Office, and two businessmen. But all four were allowed to leave after making statements concerning the money matters.

Prosecutors declined to reveal details of the questioning sessions because the case is still under probe.

Aside from the salary, the president of the nation is given a "state affairs fee" at his disposal on a monthly basis. The president does not need to present receipts or explanation for 50 percent of the expenses.

But the rules require invoices or receipts for the expenditures of the remaining half of the funds.

Prosecutors found that there were no invoices or receipts for the expenditures of over NT$50 million when Chen served as the national leader.

They suspected that the funds could have ended up in private pockets.

Chen and other members of the former first family are still under probe for suspected international money laundering operations, including depositing huge funds in other people's overseas bank accounts.

The former president made an apology to the people in August for having done something prohibited by the law. But he explained the funds abroad will be used to help the nation cultivate external relations.