Special prosecutors questioned Lee, who is now in custody, on Sunday for the second straight day on allegation that he gave or promised to offer billions of won worth of bribes to Park's jailed friend Choi Soon-sil in return for business favors.

On Feb. 17, Lee was formally arrested amid suspicion that the kickback to Choi amounted to bribes to Park. Sunday's questioning marked the fifth one since his arrest. Lee has denied the allegation.

The president is accused by the independent counsel of ordering her health minister to press the state pension fund to back a merger of two Samsung units, a key step for the smooth management succession to Lee from his ailing father Lee Kun-hee.

The deadline for the special inquiry is set for the end of this month if Acting President and Prime Minister Hwang Kyo-ahn does not approve an extension. Hwang is likely to clarify his stance over the deadline extension on Monday.

The number of suspects indicted for involvement in the scandal is expected to surpass 20, the largest-ever since 1999 when the country first introduced an independent counsel system.

In December last year, the National Assembly voted to impeach Park over the scandal which has rocked the country for months and unleashed weekly rallies against her.

The Constitutional Court is reviewing the legality of the impeachment motion and plans to listen to final arguments on Monday.

As the deadline for the independent counsel's investigation probe is nearing its end, the possibility is growing that the special prosecutors will fail to conduct face-to-face questioning of Park over the scandal.

The president earlier vowed to cooperate with the investigators' inquiry, but their previous attempts to carry out the questioning of Park have been thwarted as investigators and Park's lawyers could not narrow the gap over how and when to conduct the inquiry.

The team led by special counsel Park Young-soo said that it is likely to announce the conditional suspension of the indictment against Park when its probe ends without being extended.

The move will lead local prosecutors to decide whether to indict her at a later date if she is ousted from power. Under the country's law, incumbent presidents have legal immunity from prosecution.

Park's lawyers said that the president has yet to decide whether to appear at the final impeachment session.

The Constitutional Court is widely seen as delivering a ruling by March 13 when acting Chief Justice Lee Jung-mi is set to retire.

If Park is ousted, South Korea is required to hold a presidential election within 60 days. If she is reinstated, an election will be held in December as scheduled. (Yonhap)