Republican John Ratcliffe of Texas notes that Democratic House leader, Speaker Nancy Pelosi, has accused President Trump of "bribery".

But he points out: "No witness has used bribery to describe President Trump's conduct - none of them."

He adds: "In these 3,500 pages of sworn deposition testimony and just these 10 transcripts released thus far, the word bribery appears in these 3,500 pages exactly one time.

"And ironically it appears not in a description of President Trump's alleged conduct... but in Biden's alleged conduct."

Schiff interjects to point out it is for the impeachment investigators - not committee witnesses - to determine whether the crime of bribery has occurred.

BBC senior North America reporter Anthony Zurcher is in the hearing room...

Democrat Jim Himes also took a swipe at conservatives for insinuating that Vindman, a decorated military veteran, has dual loyalties between the US and Ukraine - the country of his birth.

He's not the only person criticising this tactic.

Former Obama campaign strategist David Axelrod is among those questioning the tactic.

Democrat Jim Himes reads out a tweet from Trump about Williams.

In the tweet, issued by the president over the weekend, Trump called Williams a "Never Trumper".

"Are you a Never Trumper?" Hines asks Williams.

I am not, she says, adding that Trump's tweet "certainly surprised me. I was not expecting to be called out by name".

"Would you call yourself a Never Trumper?" asks Connecticut Democrat Jim Himes.

The term has been used by the president as a gibe against both Vindman and his fellow witness today.

"I would call myself never partisan," Vindman replies.

Trump's just tweeted a campaign advert, which attacks the impeachment effort as a "coup".

Vindman testifies that Ukraine's national security chief once offered him the role of the country's defence minister - but he "dismissed" the offer and told his superiors about it.

"I'm an American," Vindman says. "The whole notion is rather comical."

He added that there was no concern of a conflict of interest, and his superiors never brought up the idea that there might be.

The BBC's North America reporter Anthony Zurcher notes this is some new information.

Image source, Getty Images

Ohio Republican Jim Jordan, a staunch Trump supporter in these hearings, says Vindman's colleagues felt he may have leaked material and had concerns about his judgement.

Vindman says he can't say why his national security council superior Tim Morrison questioned his judgement as they had only recently started working together.

He cites a review of his own performance, saying he was in the "top 1%".

"And you never leaked information?" Jordan asks.

Vindman says never, "that is preposterous".

A period of five-minute questioning for each lawmaker begin.

Democratic chairman Adam Schiff begins by making it clear to the public what exactly Vindman heard in that July call.

"It was pretty clear that the president wanted Zelensky to commit to investigating the Bidens?" Schiff asks.

"That is correct," Vindman says.

"That is one of the favours that you thought should be characterised as a demand?"

"That is correct."

"There's no ambiguity about that?"

"In my mind, there is not," Vindman says.

Republican counsel Steve Castor asked Vindman why he took this "very nice photo", which shows the "Three Amigos" - US officials accused of leading an alternate US foreign policy effort in Ukraine.

Those men, pictured on right, are Energy Secretary Rick Gates, US Ambassador to the EU Gordon Sondland and US special envoy to Ukraine Kurt Volker.

Vindman is asked what he remembers from the meeting and responds: "I believe it's in the deposition.

"The three elements: Burisma, Bidens, and the 2016 elections were all mentioned."

No sitting US president has ever been deposed.

So how does this process actually function?

Anthony Zurcher
BBC North America reporter

Did Alexander Vindman talk to the whistleblower about Trump’s 25 July phone call with Ukraine's president?

That certainly seems to be what Republican Devin Nunes believes – and the reason he immediately honed in on the subject upon his turn to question Vindman during Wednesday’s hearing.

When Democrat Adam Schiff cut in, saying “these proceedings will not be used to out the whistleblower”, the audience let out an audible “oooh”.

But Vindman has testified that he doesn’t know who the whistleblower is, Nunes responded, so how could he out that person?

Things got tense.

Despite being pressed, Vindman and his lawyer dug in. He would not name names.

The exchange made for what will be one of the day’s headline-grabbing moments.

Democrats have asserted that the impeachment investigation has become much bigger than the whistleblower, whose original complaint has been largely corroborated and whose identity is protected under federal law.

Republicans – from the president on down – return time and time again to the whistleblower’s identity, however.

They may believe that if they undercut that person’s credibility, the rest of the allegations will be treated with greater scepticism.

Image source, EPA

Image source, Getty Images

The whistleblower wrote to the congressional committee chairmen on 12 August expressing concern over Trump's July phone call with the Ukrainian president, and alleging that the White House had locked down details of that call.

The whistleblower did not hear the call but spoke to officials

US media have reported the individual is a male CIA officer who once worked at the White House.

The whistleblower's lawyer has said his client is willing to answer written questions submitted by House Republicans.

Trump has called the official a "disgrace to our country" and demanded the individual be named.

The identity of the alleged whistleblower has circulated widely online, though most US media have refrained from sharing the name amid legal and ethical concerns.

President Trump and his allies are fighting hard to wrest back the impeachment narrative from Democrats.

They're questioning the actions of Ukrainian politicians, as well as pushing for greater scrutiny of Joe Biden and his son Hunter.

Devin Nunes, the top committee Republican, made three specific claims in his opening statement: that the whistleblower has ties to Democrats, that Ukraine may have interfered in the 2016 election, and that the Bidens were guilty of corruption in Ukraine.

The BBC's Jonah Fisher in Kyiv & Reality Check team have fact-checked those claims.

Read the full story here.

Image source, Getty Images

Nunes asks Vindman if he discussed the 25 July call with anyone outside the White House.

The witness names State Department official George Kent and an unnamed "individual in the intelligence community".

Nunes' ears prick up, but so do Schiff's.

The Democratic committee chairman dives in, saying: "We need to protect the whistleblower. I want to make sure that there's no effort to out the whistleblower."

Vindman says: "I do not know who the whistleblower is."

He tells the committee he is not answering specific questions about members of the intelligence community under the advice of his lawyer.

Sparks fly as Vindman pulls rank.

As Republican Devin Nunes addresses the witness as "Mister Vindman", the lieutenant colonel swiftly corrects him.

"Ranking Member, it's Lt Col Vindman, please," he said.

Image source, JIM WATSON/AFP via Getty Images

Vindman says that Trump did not mention combating corruption in Ukraine on either of two calls he had with his Ukrainian counterpart.

Goldman, the Democratic lawyer, highlights that Trump's public statements suggest that the aid to Ukraine was withheld to "investigate corruption" in Ukraine.

But Vindman replies that while addressing corruption was consistent with US policy, it was not mentioned by Trump.

The Trump-Zelensky call was "segregated to a separate secure system", says Vindman, referring to a secret server where sensitive US government information is kept.

He calls the move "definitely not unprecedented".

The decision to give the call extra security, which was made by White House NSC lawyers, was made "frankly, on the fly", he says.

He said the decision was "intended to prevent leaks and access".

Some pundits have suggested this was done by White House lawyers to cover up Trump's request for a "favour" from Zelensky.

The anonymous whistleblower, who first complained about the Trump-Zelensky call, described to lawmakers how 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.

Image source, Getty Images

Here are your handy primers for all things impeachment.

SIMPLE GUIDE: If you want a basic take on what's going on, this one's for you

GO DEEPER: Here's a 100, 300 and 800-word summary of the story

WHAT'S IMPEACHMENT? It's a political process to remove a president - video guide

VIEW FROM TRUMP COUNTRY: Hear from residents in Pennsylvania

FACT-CHECK: Is the whistleblower linked to Democrats?

CONTEXT: Why Ukraine matters to the US

The president's personal lawyer, Rudy Giuliani, has taken to Twitter to condemn the impeachment effort and defend Trump.

In another tweet, he rubbished last week's witnesses, George Kent and William Taylor, who described how Giuliani led an "irregular" shadow diplomatic channel in Ukraine.

He said their testimony would not be admissible in a court of law.

Image source, Reuters

White House press secretary Stephanie Grisham says Democrats should focus on passing a trade deal instead of impeachment.

Former White House spokeswoman Sarah Sanders, now a Fox News analyst, has also been speaking out.

"If this is a TV show, it should be cancelled," she told Fox on Tuesday morning.

"No one cares. No one is paying attention because it is such a waste of time."

But according to a Reuters/Ipsos poll last week, 68% of Americans are following the hearings.

The poll also found 44% of Americans say Trump "should be impeached", while 40% said he "should not be impeached".