Prosecutors in South Korea say they believe President Park Geun-hye played a "considerable role" in an escalating corruption scandal.
The scandal, which has generated huge protests, revolves around Ms Park's relationship with an old friend, and has brought allegations of cult activities, influence-peddling and leaks of classified information.
Ms Park has so far refused to be questioned by police. Her spokesman has saidthe case against her is "imagination and speculation".
What is the relationship at the heart of the scandal?
Like many close friendships, Ms Park and Choi Soon-sil go back a long way.
In 1974, Park Geun-hye's mother was killed by a North Korean spy who had intended to kill her father, the then military leader Park Chung-hee. Ms Park, then aged 22, returned from her studies in Europe to become a stand-in first lady.Image copyrightAFP/GETTYImage captionMs Choi has returned from abroad to face questioning
It was then she got to know Choi Tae-min, a pseudo-Christian leader who set up a cult called The Church of Eternal Life. He said he had been visited by the soul of Ms Park's late mother who asked him to guide her.
He became Ms Park's mentor, while also amassing considerable wealth and power.
When Park senior was assassinated by his head of intelligence in 1979, there was speculation it was because the spy chief was worried the president was being manipulated by the man dubbed "the Korean Rasputin".
By this point Ms Park was firm friends with Mr Choi's daughter, Choi Soon-sil. Their critics believe she perpetuated her father's habits.
Like many close friendships, Ms Park and Choi Soon-sil go back a long way.
In 1974, Park Geun-hye's mother was killed by a North Korean spy who had intended to kill her father, the then military leader Park Chung-hee. Ms Park, then aged 22, returned from her studies in Europe to become a stand-in first lady.Image copyrightAFP/GETTYImage captionMs Choi has returned from abroad to face questioning
It was then she got to know Choi Tae-min, a pseudo-Christian leader who set up a cult called The Church of Eternal Life. He said he had been visited by the soul of Ms Park's late mother who asked him to guide her.
He became Ms Park's mentor, while also amassing considerable wealth and power.
When Park senior was assassinated by his head of intelligence in 1979, there was speculation it was because the spy chief was worried the president was being manipulated by the man dubbed "the Korean Rasputin".
By this point Ms Park was firm friends with Mr Choi's daughter, Choi Soon-sil. Their critics believe she perpetuated her father's habits.
Why has the friendship become problematic?
Few claims have been off-limits in the media coverage of the scandal, with some reports going as far as suggesting the president is a puppet who hosted shamanist rituals at the presidential compound. But many of the lurid claims are unsubstantiated.Image copyrightREUTERSImage captionWith no immediate family, President Park came to rely too much on her friend, critics claim
The official investigation focuses on Ms Choi's alleged abuse of her closeness to the president to enrich herself and influence policy, as well as her handling of classified documents.
Ms Choi is also alleged to have used her presidential connections to pressure companies for millions of dollars in donations to two non-profit foundations she controlled. The claims have even swept up Samsung in the investigation.
President Park is alleged to have been personally involved, instructing Ms Choi and two presidential aides to collect money for the launch of Ms Choi's foundations, according to prosecution documents submitted to the court.
Ms Choi is also accused of having received large numbers of confidential government documents from Ms Park, via an aide. These allegedly included information about ministerial candidates and North Korea.
There are even claims Ms Choi took advantage of the president's wardrobe budget - buying cheap outfits and keeping the change.Image copyrightAFPImage captionProtesters have marched in vast numbers to demand Ms Park stands down
On Sunday 20 November, Ms Choi was formally charged with various offences, including abuse of authority, coercion, attempted coercion and attempted fraud. Two former presidential advisers were also charged by prosecutors, who said they thought the president conspired in the wrongdoing.
The allegations are strenuously denied by Ms Park.
Few claims have been off-limits in the media coverage of the scandal, with some reports going as far as suggesting the president is a puppet who hosted shamanist rituals at the presidential compound. But many of the lurid claims are unsubstantiated.Image copyrightREUTERSImage captionWith no immediate family, President Park came to rely too much on her friend, critics claim
The official investigation focuses on Ms Choi's alleged abuse of her closeness to the president to enrich herself and influence policy, as well as her handling of classified documents.
Ms Choi is also alleged to have used her presidential connections to pressure companies for millions of dollars in donations to two non-profit foundations she controlled. The claims have even swept up Samsung in the investigation.
President Park is alleged to have been personally involved, instructing Ms Choi and two presidential aides to collect money for the launch of Ms Choi's foundations, according to prosecution documents submitted to the court.
Ms Choi is also accused of having received large numbers of confidential government documents from Ms Park, via an aide. These allegedly included information about ministerial candidates and North Korea.
There are even claims Ms Choi took advantage of the president's wardrobe budget - buying cheap outfits and keeping the change.Image copyrightAFPImage captionProtesters have marched in vast numbers to demand Ms Park stands down
On Sunday 20 November, Ms Choi was formally charged with various offences, including abuse of authority, coercion, attempted coercion and attempted fraud. Two former presidential advisers were also charged by prosecutors, who said they thought the president conspired in the wrongdoing.
The allegations are strenuously denied by Ms Park.
What do the two women say?
When she was first questioned in October, Ms Choi said she had committed an "unpardonable crime", though her lawyer said this was not a legal admission of guilt.
President Park has herself admitted some lapses. She says she did consult Ms Choi for advice, and that she helped her edit her speeches, but that this stopped once she had a team of advisers in place.
Witnesses have claimed that Ms Choi received briefings and official papers long after that occurred. Documents were also discovered on an unsecured tablet computer found in an old office of Ms Choi's.
But the tone of the president's pronouncements has changed over time. She began with opaque apologies: "Regardless of what the reason may be, I am sorry that the scandal has caused national concern and I humbly apologise to the people."Image copyrightAFPImage captionMs Park said she had "put too much faith" in her friendship with Ms Choi
But she has moved on to "heartbroken" public confessions of naivety: "Sad thoughts trouble my sleep at night. I realise that whatever I do, it will be difficult to mend the hearts of the people, and then I feel a sense of shame."
She had said she was willing to be questioned by investigators, but has so far resisted their attempts to speak to her.
Her spokesman said the prosecutors' allegation that she colluded with Ms Choi was "deeply regrettable".
"The announcement is not the truth at all and but a house of cards built on repeated imagination and speculation that completely ignores objective evidence," said Jung Youn-kuk.
When she was first questioned in October, Ms Choi said she had committed an "unpardonable crime", though her lawyer said this was not a legal admission of guilt.
President Park has herself admitted some lapses. She says she did consult Ms Choi for advice, and that she helped her edit her speeches, but that this stopped once she had a team of advisers in place.
Witnesses have claimed that Ms Choi received briefings and official papers long after that occurred. Documents were also discovered on an unsecured tablet computer found in an old office of Ms Choi's.
But the tone of the president's pronouncements has changed over time. She began with opaque apologies: "Regardless of what the reason may be, I am sorry that the scandal has caused national concern and I humbly apologise to the people."Image copyrightAFPImage captionMs Park said she had "put too much faith" in her friendship with Ms Choi
But she has moved on to "heartbroken" public confessions of naivety: "Sad thoughts trouble my sleep at night. I realise that whatever I do, it will be difficult to mend the hearts of the people, and then I feel a sense of shame."
She had said she was willing to be questioned by investigators, but has so far resisted their attempts to speak to her.
Her spokesman said the prosecutors' allegation that she colluded with Ms Choi was "deeply regrettable".
"The announcement is not the truth at all and but a house of cards built on repeated imagination and speculation that completely ignores objective evidence," said Jung Youn-kuk.
Is anyone else involved?
Several former presidential aides have been investigated in connection with the case, with two of them now charged.
An Chong-bum, Ms Park's former senior secretary for policy co-ordination, has been charged with abuse of authority, coercion and attempted coercion, while Jung Ho-sung is accused of passing classified presidential documents to Ms Choi.
Local media has also been busy finding colourful associates of Ms Choi that were close to the president, including various celebrities and her personal trainer, who was appointed as a presidential aide.
Several former presidential aides have been investigated in connection with the case, with two of them now charged.
An Chong-bum, Ms Park's former senior secretary for policy co-ordination, has been charged with abuse of authority, coercion and attempted coercion, while Jung Ho-sung is accused of passing classified presidential documents to Ms Choi.
Local media has also been busy finding colourful associates of Ms Choi that were close to the president, including various celebrities and her personal trainer, who was appointed as a presidential aide.
What now?
South Korea's constitution does not allow a sitting president to be prosecuted and she has 15 months left in her term.
But now that prosecutors have directly linked Ms Park to the scandal, it is possible she could be impeached for breaking the law. Opposition parties, including the main People's Party, say this is what they will try to do.
It will take a two-thirds majority vote in parliament to unseat her, which is possible if enough of her own MPs support the move.
If they do, it will fall to the Constitutional Court to rule on the matter. If six of nine judges agree, she can be removed from office.
If she is unseated or steps down, as hundreds of thousands of Koreans are demanding, elections must then be held within 60 days.
South Korea's constitution does not allow a sitting president to be prosecuted and she has 15 months left in her term.
But now that prosecutors have directly linked Ms Park to the scandal, it is possible she could be impeached for breaking the law. Opposition parties, including the main People's Party, say this is what they will try to do.
It will take a two-thirds majority vote in parliament to unseat her, which is possible if enough of her own MPs support the move.
If they do, it will fall to the Constitutional Court to rule on the matter. If six of nine judges agree, she can be removed from office.
If she is unseated or steps down, as hundreds of thousands of Koreans are demanding, elections must then be held within 60 days.
