South Korea's President Park Geun-hye says she is "heartbroken" over a scandal threatening her leadership.
She said she took sole responsibility for her close friend Choi Soon-sil's access to government documents and was willing to be investigated.
Ms Choi is suspected of using her friendship with Ms Park to solicit business donations for a non-profit fund she controlled.
She was detained on Monday. A former aide to Ms Park was held on Thursday.
On Thursday, a spokesman for Seoul Central District Court said it had accepted a request from prosecutors to issue an arrest warrant for Ms Choi, on charges of fraud and abuse of power.
Prosecutors have also said they are expanding their official investigation.
'Sad thoughts trouble my sleep'
The scandal has left Ms Park with an approval rating of just 5%.
She has already replaced her prime minster, reshuffled her cabinet and dismissed several aides, but there are growing calls for her resignation or impeachment.
Ms Park has admitted letting Ms Choi help her with speeches, without security clearance, while there are also reports that Ms Choi was closely involved in forming government policy.
In a brief televised address on Friday, an emotional Ms Park again apologised, saying she "put too much faith in a personal relationship and didn't look carefully at what was happening".
"Sad thoughts trouble my sleep at night. I realise that whatever I do, it will be difficult to mend the hearts of the people, and then I feel a sense of shame and ask myself, 'Is this the reason I became president?'"
She said anyone found to have done wrong would be punished, and "if necessary, I'm determined to let prosecutors investigate me and accept an investigation by an independent counsel too".
But she denied speculation that her presidency had been influenced by a cult or that shamanistic rituals had been held at the presidential compound.
Ms Choi, a long-time friend of Ms Park's, is the daughter of Choi Tae-min, a shadowy quasi-religious leader who was closely linked to Ms Park's father, then-president Park Chung-hee.