Ahn Jong-beom, a former aide to South Korean President Park Geun-hye, has been placed under emergency detention, prosecutors say.
He is the second aide to be detained in a scandal which is threatening Park Geun-hye’s position.
Ahn Jong-beom and Park Geun-hye’s long-time friend and mentor Choi Soon-sil are suspected of using their presidential connections to persuade corporations to donate funds to foundations they were involved with.
Ahn Jong-beom had told reporters on November 2 he would take responsibility for his actions, the Yonhap news agency reports, without giving further details.
He has been placed under emergency detention to prevent the possibility of him tampering with evidence, prosecutors said. They have 48 hours to formally request an arrest warrant in court.
On November 2, prosecutors requested an arrest warrant for Choi Soon-sil. Seoul Central District Court is due to review the request on November 3, a court spokesman said.
Several banks have also been raided in connection with the controversy, which has prompted widespread anger.
Park Geun-hye responded to the crisis on November 2 by reshuffling her government, appointing a new finance minister and prime minister – a largely symbolic role in South Korea.
Surveys suggest Park Geun-hye’s approval rating is about 10%, with about half of respondents saying she should resign or face impeachment.