US Secretary of State Mike Pompeo has been ordered by Democrats to turn over documents relating to the Trump administration’s dealings with Ukraine.
In a letter, the heads of three House committees subpoenaed Mr Pompeo to produce the documents within a week.
It is the latest move in rapidly escalating impeachment proceedings against President Donald Trump.
He is being scrutinised for allegedly pressuring Ukraine’s president to investigate Democratic rival Joe Biden.
In a separate development on Friday, the US special envoy for Ukraine negotiations, Kurt Volker, resigned, US media reported.
Mr Trump has denied putting any pressure on Ukrainian President Volodymyr Zelensky in a phone call in July, when Mr Biden was leading polls to win the Democratic nomination for the White House race in 2020.
Mr Trump has alleged that Mr Biden pressed for the sacking of Ukrainian prosecutor Viktor Shokin in 2016 to protect a business that employed his son, Hunter Biden.
Mr Biden did call for the sacking of Mr Shokin, even threatening to withhold $1bn (£813m) in aid to Ukraine. But so did a number of other Western officials who saw Mr Shokin as a hindrance to anti-corruption investigations.
Impeachment is a two-stage political process, rarely exercised, by which a US president can be removed from office for wrongdoing.
Even if President Trump were impeached by the House of Representatives, he is unlikely to be forced out of the White House because Republicans control the Senate.
What is the Pompeo subpoena about?
It was issued in the joint letter by the House’s Foreign Affairs, Intelligence and Oversight committees.
The committees are headed by Elliot Engel, Adam Schiff and Elijah Cummings respectively.
The letter says the subpoena was issued because Mr Pompeo had failed to comply with the demand to provide requested documents.
“Your continued refusal to provide the requested documents not only prevents our committees from fully investigating these matters, but impairs Congress’ ability to fulfil its constitutional responsibilities to protect our national security and the integrity of our democracy,” the letter said.
The three committees also informed Mr Pompeo that they planned to request testimonies from five officials, including Mr Volker and the former US ambassador to Ukraine, Marie Yovanovitch, in the next two weeks.
Mr Pompeo has so far made no public comments on the subpoena.
What is the claim about Joe Biden?
On 25 July, Mr Trump raised Mr Shokin’s removal during a phone call with newly elected President Zelensky, details of which were released by the White House this week after a huge row over a complaint by a whistleblower.
Mr Trump went on to discuss Hunter Biden and the unsubstantiated allegation that his father – then the US vice-president – stopped an investigation into his son’s employer by lobbying Ukraine to fire Mr Shokin.
The chief prosecutor’s office was inquiring into Burisma, a natural gas company on which Hunter Biden was a board member.
There is no evidence of any wrongdoing by the Bidens.
Who is Kurt Volker?
Appointed in 2017 in a volunteer capacity, Mr Volker was a key player in US efforts to help resolve an ongoing crisis in Ukraine that started with the annexation of Crimea by Russia and Moscow’s support for separatists in the east.
Mr Volker was mentioned in the whistleblower’s complaint on 12 August.
It says that Mr Volker and US Ambassador to the EU, Gordon Sondland, met President Zelensky and other Ukrainian politicians on 26 July.
The complaint says Mr Volker and Mr Sondland “reportedly provided advice to the Ukrainian leadership about how to ‘navigate’ the demands that [President Trump] had made of Mr Zelensky”.
It says that Mr Volker and Mr Sondland had also spoken with Rudi Guiliani, Mr Trump’s lawyer, to try to “contain the damage” to US national security.
Why is the July phone call controversial?
Democrats accuse Mr Trump of illegally seeking foreign help in the hope of smearing Mr Biden.
The US president called on the Ukrainian leader to talk to US Attorney General William Barr and Mr Trump’s lawyer Rudy Giuliani about investigating Hunter Biden’s past business dealings.
Mr Trump is accused of using military aid to Ukraine as a bargaining tool. The package – which has since been released – was aimed at supporting US interests in a friendly country.
Mr Trump, a Republican, denies any wrongdoing and has dismissed the impeachment proceedings as a “hoax” and “another witch-hunt”.
He acknowledged that he had personally blocked nearly $400m (£324m) in military aid to Ukraine days before he spoke to Mr Zelensky, but denied that it was to pressure the Ukrainian leader into investigating Mr Biden.
Who is the whistleblower and what was said?
Few details are known. A lawyer for the whistleblower warned that trying to identify the person could place them “in harm’s way”.
The New York Times, Washington Post and Reuters news agency identified the whistleblower as a CIA officer.
Their complaint accuses Mr Trump of “using the power of his office to solicit interference from a foreign country in the US 2020 election”.
The declassified document characterises the president’s conduct as a “serious or flagrant problem, abuse, or violation of law”.
The whistleblower says they had learned from several sources that senior White House officials had intervened to “lock down” all records of the call, particularly an official word-for-word transcript.
“This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call,” the whistleblower wrote in the complaint.