WASHINGTON (WASHINGTON POST, NYTIMES, REUTERS) - An anonymous whistleblower complaint last week on US President Donald Trump has sparked an impeachment inquiry into the US leader.
The complaint was prompted by Mr Trump's move to get his Ukrainian counterpart Volodymyr Zelensky to launch an investigation that could damage his Democratic political rival Joe Biden.
Here are some of the major players in the political maelstrom.
During a July phone call, US president Donald Trump asked his Ukrainian counterpart Volodymyr Zelensky - whose country is dependent on US military aid to combat a Russia-backed insurgency - to investigate Democratic rival Joe Biden and his son, Hunter. At around the same time, Mr Trump blocked US400 million (S$553 million) aid to Ukraine.
A whistleblower filed a complaint over the contents of the conversation in August, saying Mr Trump had tried to get a foreign power to interfere in a US election for his own gain.
Mr Volodymyr Zelensky is a former comedian-turned-politician who was elected Ukraine's president in April. Despite being in the middle of a political storm, Mr Zelensky has insisted he had not been "pushed" by Mr Trump to do anything.
Joe Biden & Hunter Biden
Former vice-president Joe Biden is considered the frontrunner for the Democratic nomination for the 2020 presidential election.
Hunter, 49, is the son of Joe Biden. Hunter joined the board of Burisma Group, a private company that has contracts to drill for natural gas in Ukraine, in May 2014, while his father was still vice-president. He left the company in April this year.
Around that time, Ukraine's senior prosecutor was probing Burisma but was later removed. Mr Trump and his allies have raised the spectre of impropriety on the part of Mr Biden.
In May this year, Ukraine's then prosecutor Yuri Lutsenko said he had no evidence of wrongdoing by Hunter Biden or his father.
Back when he was vice-president, Mr Joe Biden and other Western leaders pressured Ukraine to get rid of the country's top prosecutor, Mr Viktor Shokin, because he was seen as an obstacle to reform as well as not tough enough on corruption.
However, Mr Trump has claimed that Mr Biden pressured the Ukrainian government to fire Mr Shokin because he was investigating Burisma. But at that time, the investigation into Burisma was dormant, according to former Ukrainian and US officials.
Mr Yuri Lutsenko was Ukraine's former top law enforcement official who took over after Mr Shokin was fired. He recounted yet again the efforts by Mr Trump to pressure him to investigate the Biden family. In an interview in Kiev with the Los Angeles Times, Mr Lutsenko said he told Trump lawyer Rudy Giuliani that there was no evidence that Mr Biden or his son had broken Ukrainian laws.
Mr Rudolph Giuliani is Mr Trump's personal lawyer and is at the centre of the effort to get Ukraine to conduct a probe of the Bidens.
His name was mentioned numerous times on the whistleblower's report and in recent weeks, he has admitted in interviews to trying to gather damaging information in Ukraine on Democrats, including Mr Biden, that would help the president politically.
Three House committees said they had subpoenaed Mr Giuliani for records of his dealings with Ukraine on Mr Trump's behalf.
Last Friday, the House Foreign Affairs Committee issued a subpoena to Secretary of State Mr Pompeo for documents related to the Ukraine scandal after news emerged that he was among the government officials listening in on the July call between Mr Trump and Mr Zelensky.
House Speaker Nancy Pelosi, a Democrat, announced on Sept 24 that the US House of Representatives has launched an official impeachment investigation into Mr Trump. The probe will centre on whether Mr Trump abused his presidential powers and sought help from a foreign government to undermine Mr Biden and help his own re-election.
The Democrat leading the impeachment investigation, Adam Schiff is the chairman of the House Intelligence Committee. His committee also looked into Russian inteference in the 2016 presidential election last year.
The identity of the whistleblower is not known. But what is clear is that on Aug 12, the whistleblower, said to be from within the intelligence community, lodged a complaint with an internal watchdog that reportedly focused on Mr Trump's contacts with Ukraine.
The complaint has since snowballed into an impeachment inquiry into the US leader.