South Korean prosecutors Thursday questioned three former and incumbent senior executives of SK Group, renewing a push to investigate the alleged corrupt ties between ousted President Park Geun-hye and big businesses.

The three, including Kim Chang-geun, former chairman of SK’s top decision-making body SUPEX Council, showed up at the Seoul Central District Prosecutors’ Office in southern Seoul at 10 a.m. to be questioned over allegations that the group bribed Park and her jailed friend Choi Soon-sil in exchange for business favors.

SK, the country’s third-largest business group, donated 11.1 billion won ($9.60 million) to two nonprofit foundations controlled by Choi between 2015 and 2016, which the investigators suspect were kickbacks for a presidential pardon in August 2015 for group Chairman Chey Tae-won who was serving a jail term for embezzlement.

SK denies the bribery. Asked by reporters on his way to the prosecutors’ office, Kim also denied the allegations.

The summoning of SK executives comes just a day after the prosecution sent out a summons to former President Park, who was sacked last week by the Constitutional Court over a sweeping corruption scandal involving Choi, her friend of 40 years.

Investigations so far, including one led by an independent counsel, had focused on Park-Choi ties and their alleged shady dealings with Samsung Group using the aforementioned foundations. Samsung Group de facto leader Lee Jae-young is currently under arrest, pending a criminal trial for bribery.

Prosecutors plan to expand their probe into other conglomerates, including the Lotte and CJ groups, which also gave donations to Choi’s foundations, a prosecution official told reporters.

“We will take due action on them when necessary,” he said.

As for SK Group chief Chey, the official did not rule out the possibility of summoning him for questioning. “We will look into that later,” he said.

SK Group’s business spans from telecommunication to oil refinery, with the flagship units SK Hynix, the world’s second-largest memory chip maker, and SK Telecom, the country’s No. 1 telecom firm.

Prosecutors also suspect that aside from Chey’s pardons, the donations were in relation to the group’s winning of a duty-free license.

Independent counsel Park Young-soo, upon concluding his 70-day investigation, booked then-President Park as an accomplice to Choi in the bribery. He said Park “shared economic interests” with Choi and they ran the foundations together.

The state prosecution is to question Park in person Tuesday.

A total of 53 companies, including Samsung and SK, gave a total of 77.4 billion won ($66.9 million) in donations to the Mir and K-Sports foundations.

By Bak Se-hwan (sh@heraldcorp.com)