Democrats in the US House of Representatives have published a resolution setting out the next steps in their impeachment efforts against President Donald Trump.
The motion sets out a more public phase of the inquiry and hands the lead role in hearings to the chairman of the intelligence committee, Adam Schiff.
The House, controlled by the Democrats, will vote on the measure on Thursday.
A White House spokeswoman said the resolution was an "illegitimate sham".
So far, hearings have been held behind closed doors. This vote to make the impeachment process public is about the procedure, and not a ballot on whether or not to impeach the president.
Republicans have criticised Democrats for the closed hearings up to this point, in which Republican lawmakers have also taken part. But Democrats insist they were needed to gather evidence ahead of the public stage of the inquiry, and deny allegations they have been secretive.
President Trump is accused of trying to pressure Ukraine into investigating unsubstantiated corruption claims against his political rival, Joe Biden, and his son who worked with Ukrainian gas company Burisma.
Mr Trump denies wrongdoing and calls the impeachment inquiry a "witch hunt".
On Tuesday, the impeachment inquiry heard from Lt Col Alexander Vindman, a White House official who had monitored a phone call on 25 July between Mr Trump and Ukrainian President Volodymyr Zelensky.
That call sparked a whistleblower complaint and led to the impeachment probe.
Col Vindman said he was "concerned" by the call as he "did not think it was proper to demand that a foreign government investigate a US citizen".
Some details of the call were omitted from the official transcript, despite his attempts to have them included, Col Vindman added, according to US media citing sources.
What does the resolution say?
The eight-page document sets out a two-stage process for the next phase of the inquiry.
In the first, the House Intelligence Committee will continue its investigations and hold public hearings. It will have the right to make public transcripts of depositions taken in private.
In the second phase, a public report on the findings will be sent to the House Judiciary Committee which will conduct its own proceedings and report on "such resolutions, articles of impeachment, or other recommendations as it deems proper".
President Trump's lawyers will be allowed to take part in the Judiciary Committee stage.
Republicans on the committees will be able to subpoena documents or witnesses - although they could still be blocked as both committees are Democrat-controlled.
House Speaker Nancy Pelosi has said a House vote on the resolution would take place on Thursday. She has previously said such a vote is not required under the US Constitution.
What do Mr Trump's supporters say?
"The resolution put forward by Speaker Pelosi confirms that House Democrats' impeachment has been an illegitimate sham from the start as it lacked any proper authorisation by a House vote," said White House spokeswoman Stephanie Grisham.
House Republican leader Kevin McCarthy, speaking before the resolution was unveiled, said the entire process was a "sham."
Referring to the closed-door meetings and depositions he said: "You can't put the genie back in the bottle. Due process starts at the beginning."
What did Col Vindman say?
In an opening statement released ahead of Tuesday's closed-door testimony, Col Vindman said his worries began at a 10 July meeting between US and Ukrainian national security officials.
The meeting was cut short by then-National Security Adviser John Bolton when talk arose of Ukraine opening investigations for the White House, Col Vindman said.
US ambassador Gordon Sondland had brought up "Ukraine delivering specific investigations in order to secure [a] meeting with the president", which alarmed Mr Bolton and Col Vindman.
At a debriefing afterwards, Col Vindman said the ambassador had again "emphasised the importance that Ukraine deliver the investigations into the 2016 election, the Bidens and Burisma".
"I stated to Ambassador Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security."
Following this incident, Col Vindman reported his concerns to the National Security Council's lead counsel. He reported his objections again after the 25 July call.
Col Vindman said he attempted to edit a White House transcript of the call to include some missing details, US media cite sources as saying.
President Trump has previously described the transcript as "exact".
What was the reaction?
As the testimony was due to begin, Mr Trump suggested Col Vindman was a "Never Trumper witness" in a tweet.
Other conservatives have also attacked Col Vindman's credibility because he was born in Ukraine - though some have since defended the veteran officer.
Supposedly, according to the Corrupt Media, the Ukraine call “concerned” today’s Never Trumper witness. Was he on the same call that I was? Can’t be possible! Please ask him to read the Transcript of the call. Witch Hunt!— Donald J. Trump (@realDonaldTrump) October 29, 2019
Republican congresswoman Liz Cheney denounced these gibes as "shameful", while Senator Mitt Romney called the criticism "absurd" and "disgusting".
"This is a decorated American soldier and he should be given the respect that his service to our country demands," he said.
But many other Republicans stood by Mr Trump. House Minority leader Kevin McCarthy said Col Vindman was "wrong".
"Nothing in that phone call is impeachable," he said.
Quick facts on impeachment
Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office.
If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial.
A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber.
Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached, but neither was convicted and removed.
President Richard Nixon resigned before he could be impeached.