Besides reviewing the president's speech drafts and confidential documents without any official government position and abusing her influence, the 60-year-old is suspected of getting preferential treatment from local banks for loans and other financial transactions.

Hours after Choi appeared before prosecutors to face questioning on Monday, authorities raided the headquarters of eight banks -- Woori Bank, Standard Chartered Bank Korea, Shinhan Bank, KB Bank, KEB Hana Bank, Industrial Bank of Korea, Citibank Korea and Nonghyup Bank -- to confiscate documents related to her financial transactions.

Choi is known to have taken 500 million won ($437,675) in loans from KB Bank using her land in PyeongChang in the eastern province of Gangwon as collateral, industry officials with knowledge of the matter said.

According to an opposition lawmaker, Choi and her 20-year-old daughter also received a loan of around 250,000 euros ($270,000) from KEB Hana Bank by using their land in PyeongChang in late 2015, just months before moving to Europe.

KEB is currently under investigation by the Financial Supervisory Service, a financial watchdog.

The investigators also asked for financial documents of Cha Eun-taek, a well-known advertisement director and Choi's close aide as he is under suspicion of meddling in government's cultural projects, they noted.

Choi, who returned from her two-month stay in Germany on Sunday, was placed under detention late Monday.

Choi's summons comes less than a week after prosecutors raided the offices of Mir Foundation and K-Sports Foundation, which are suspected of raising massive funds from conglomerates with the help of the Federation of Korean Industries (FKI), a major business lobby group.

Choi is under suspicion of playing the role of the behind-the-scenes power broker by forcing conglomerates to donate money to the funds using her connection with presidential aides and high-ranking officials.

According to the data compiled by corporate tracker Chaebul.com, a total of 53 companies donated a combined 77.4 billion won to Mir Foundation and K-Sports Foundation, which were established in 2015 and early this year, respectively.

Among them, 12 companies gave out money to the foundations although they logged deficits last year, raising speculations about whether their donations were voluntary as claimed by foundation officials.

Hyundai Motor Co. donated the biggest amount of 6.88 billion won, followed by SK Hynix Co. with 6.8 billion won and Samsung Electronics Co. with 6 billion won.

Some companies didn't record the donations in their regulatory briefings, sparking questions over the source of the funds and violation of accounting rules. (Yonhap)