Two senior executives of South Korea's largest business group Samsung were questioned by investigators Monday as they accelerated the probe into allegations surrounding the influence-peddling scandal that has led to President Park Geun-hye's impeachment.

Choi Gee-sung, a vice chairman, and Chang Choong-ki, a president at the conglomerate, appeared in the office of the independent counsel team. They lead the group's future strategy planning division believed to be the de facto control tower that makes key decisions for Samsung and is closely linked to the ongoing scandal.

Samsung is suspected of giving financial support to foundations and a company controlled by Park's friend Choi Soon-sil, who is at the center of the scandal, in return for a state-run pension fund's backing of a major merger deal between Samsung subsidiaries in 2015.

The probe team is investigating whether the presidential office Cheong Wa Dae pressured the state fund to support the merger in return for favors Samsung had given to Park's friend, who is standing trial over a string of corruption charges.

"The two were called in to bear witness to the case, but their statuses could change to that of suspect in the course of the interrogation," an official on the investigation team said.

On the same day, the probe team summoned Choi Soon-sil, but she declined to appear, saying she needs to prepare for her own trial slated for Wednesday and President Park's impeachment hearing Tuesday.

The Constitutional Court reviewing Park's impeachment designated Choi as a witness to appear at the court Tuesday.

She has rejected previous summons by the investigators, citing health issues.