The woman suspected of exerting influence over South Korean President Park Geun-hye for personal gain has apologised for an "unpardonable crime".
Choi Soon-sil, the daughter of a cult leader, spoke amid a throng of media and protesters as she entered the prosecutor's office in Seoul.
Ms Choi is said to have been involved in government business despite having no security clearance.
Ms Park has acknowledged that Ms Choi edited some of her speeches.
A 'feeding frenzy' - BBC's Steve Evans at the scene
Scrums don't come much bigger. Hundreds of cameras and reporters swarmed around the tiny figure of Choi Soon-sil, a black hat down low over her eyes. In this melee, she apparently apologised saying she had committed an unpardonable crime - without specifying what it might be. But the real political traction comes from her relationship with the country's leader.
It is said that President Park consulted Ms Choi on minor matters like what colours to wear and on large matters of policy.
Prosecutors are also investigating allegations of financial corruption. The media and opposition smell blood. There is a feeding frenzy.
Ms Choi is the daughter of shadowy religious cult leader, Choi Tae-min, who was Ms Park's mentor until his death in 1994.
The scandal has badly eroded the president's popularity before next year's elections. Thousands of people took to the streets on Saturday to demand she resign.
Choi Soon-sil flew from Germany to Seoul on Sunday.
"Please, forgive me," a teary Ms Choi said at the prosecutor's office, according to Yonhap news agency.
However, her lawyer denied she was admitting guilt.
"It wouldn't be right to take it as any kind of legal statement," Lee Kyung-jae told reporters.
According to media reports, Ms Choi pressured businesses to donate some 80bn won ($70m; £57m) to two non-profit organisations she ran, using leverage provided by her close relationship with the president.
She is alleged to have in turn personally misappropriated some of these funds.
A televised apology from Ms Park last week failed to defuse the situation. The president has said that Ms Choi was someone who "gave me help when I was going through a difficult time".
Opposition politician Jae-myung Lee said Ms Park had "lost her authority as president".
On Saturday, prosecutors raided the homes of several presidential aides, seizing computers and files belonging to the officials who are suspected of being Ms Choi's accomplices.
Ms Park, 64, became the first woman to lead South Korea after winning presidential elections in 2012.