South Korean prosecutors say they believed President Park Geun-hye was an accomplice in a corruption scandal that has rocked her administration, in a heavy blow to her fight for political survival.
The prosecutors' comments, which came on Sunday as they indicted a close friend of Park's and two of her former aides, are likely to spur stronger calls for her to step down or be impeached.
Park's close friend Choi Soon-sil and former presidential aide An Chong-bum were charged with abuse of power in pressuring companies to contribute funds to foundations at the centre of the scandal, said Lee Young-ryeol, head of the Seoul Central District Prosecutors' Office.
"The special investigation team concluded that based on the evidence secured to date, the president was in complicity with Choi Soon-sil, An Chong-bum and Jeong Ho-seong to a considerable degree," Lee told a news conference.
Jeong, also one of Park's former aides, was indicted for leaking classified information to Choi.
Park cannot be indicted because she has constitutional immunity, Lee said, but added: "We will continue to investigate the president," without elaborating.
Under the constitution, a sitting president cannot be indicted unless on charges of treason, but the conclusion by the prosecutors that Park was involved in the case prompted fresh calls from opposition parties for her to step down.
The main opposition Democratic Party and the centrist People's Party said Park will face impeachment proceedings if she refuses to resign. But they stopped short of saying they would immediately initiate such a move.
Analysts said the prosecutors' comments about the president's involvement increased the prospect that she would face impeachment.
"It provided a legal basis for impeachment proceedings, not only her moral and political liabilities," said Kim Jun-seok, a political science professor at Dongguk University in Seoul.
Park is unlikely to voluntarily step down because she would lose immunity against prosecution, Kim said. Her five-year term ends in February 2018.
"Then, the only option that is left for politicians given the worsening public sentiment is impeachment," he said.
South Korea's presidential Blue House did not have an immediate comment. Lee did not say when the prosecutors will question Park.
Park's lawyer Yoo Yeong-ha said the prosecutors' announcement was "deeply regrettable," according to South Korean media. Yoo did not respond to telephone calls or text messages from Reuters seeking comment.
Park has resisted calls to resign over the scandal but has publicly apologised twice, saying that it was caused by her shortcomings and that she had sought help from the business community in the belief it would benefit the economy, not for personal gain.