SEOUL, South Korea (AP) — Despite her immunity from prosecution, investigators can look into South Korea's president over a scandal that centers on whether her longtime friend controlled the government from the shadows, the prime minister-designate said Thursday.

The comments by Kim Byong-joon, who President Park Geun-hye nominated amid rising public anger, came as opposition lawmakers and activists stepped up demands for prosecutors to directly investigate Park to get to the bottom of a scandal that has plunged the country into political turmoil. One recent opinion survey showed Park's approval rating at about 9 percent.

South Korean media speculate that Park's friend, Choi Soon-sil, who is not officially in the government, manipulated government affairs and pushed businesses to donate millions of dollars to two foundations that she controlled, although she didn't have any government post.

On Thursday, Kim told a televised conference that he thinks it's possible to have Park investigated because "all nationals are equal before the law." But he said the procedures and methods of any probe of the head of state must be carefully done.

Kim said there are different interpretations of a constitutional provision that grants sitting presidents immunity from criminal lawsuits unless they are accused of serious crimes such as treason. He didn't elaborate, but his comments likely reflected a view by some scholars that the constitution disallows an indictment and other legal steps beyond an investigation or questioning of the president.

Kim, a public administration professor, served as a top policy adviser for late liberal President Roh Moo-hyun. Park's nomination of Kim for the country's No. 2 job was seen as an effort to reach out to liberals for bipartisan support.

But the prospect of parliamentary approval for Kim's nomination is unclear as the main opposition Democratic Party described it as a tactic to divert attention from the scandal.

Park, a conservative, may survive what has become the worse patch of an already rocky four years in office. But if her choice for prime minister is rejected, and she is forced to name someone chosen by the opposition, it will hamstring her authority and may end her ability to govern.

On Wednesday night, prosecutors detained a former senior aide of Park after summoning him over his alleged involvement in extracting $70 million in donations. He is the second person detained in connection with the scandal. Earlier this week, prosecutors detained Choi and requested an arrest warrant for her.

Much of the public frenzy over the scandal is associated with Choi's family background. Her father led a religious cult and reportedly was a private mentor for Park, whose parents each were assassinated in the 1970s. Park's father was a military dictator who ruled South Korea for 18 years.

While acknowledging her ties to Choi Soon-sil last week, Park said Choi helped her "when I had difficulties" in the past. Park acknowledged that Choi had edited some of her speeches and provided public relations help. South Korean media speculate Choi may have had access to sensitive information and played a much larger role in government affairs.

Park has already been criticized for an aloof manner and for relying on only a few longtime confidantes. That she may have outsourced sensitive decisions to someone outside of government, and someone connected with a murky, lurid backstory, has incensed many.