Media playback is unsupported on your device Media caption Key moments from Trump impeachment hearing

Going into the first day of public impeachment hearings, it seemed as though details of the proceedings would be relatively familiar for those who have followed the last month of coverage of closed-door testimony from a parade of current and former government officials.

Toward the end of his opening statement, however, acting US Ambassador to Ukraine Bill Taylor changed the script.

He revealed that one of his aides had been with US Ambassador to the EU Gordon Sondland as he carried out a phone conversation with Donald Trump following a meeting with Ukrainians on 26 July, the day after the now-famous Trump-Zelensky phone call.

According to Taylor, his aide heard Trump ask about "the investigations" - and Sondland replied that Ukraine was ready to move forward.

Sondland then told Taylor's aide that the president cared more about the investigation of the Bidens than anything else involving Ukraine.

This has the potential to be a major twist.

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There's been considerable testimony about Sondland's interaction with the Ukrainians, and the ambassador himself has testified that he told Ukrainian officials on 1 September that he presumed US military aid was probably being withheld until they opened investigations.

Media playback is unsupported on your device Media caption A Democrat and a Republican react to the first public impeachment hearing

During his Wednesday appearance, Taylor spoke of how Sondland told him that the president, as a businessman, wanted what was "owed him" before he "signed the cheque" and that there would be a "stalemate" if Ukraine didn't act - which Taylor interpreted as meaning military aid would not resume without investigations.

Although there have also been reports of Sondland's direct line to the president, there has yet to be evidence tying Mr Trump directly to the alleged quid pro quo.

The phone call Taylor described could change all that.

In the middle of Wednesday's hearing, the House Intelligence Committee announced a new witness scheduled to give a closed-door deposition on Friday, an aide to Taylor named David Holmes - reportedly the aide Taylor mentioned.

Learn more about Trump and impeachment inquiry

Next week, Sondland himself is scheduled to testify during public hearings.

Although the ambassador did not mention the phone call with the president in his original closed-door testimony, he's already had to update that testimony to reflect a new recollection of his discussions with Ukrainian officials about military aid. Democrats may be hoping he does so again.

If either Sondland or Holmes supports Taylor's account, it could undercut the president's defenders who have suggested that Mr Trump was not closely involved in the activities of the "unofficial" channel of Ukraine policy, as Taylor called it, which was pressuring Ukraine to open up investigations into the Bidens.

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Just last week, when asked about his relationship with Sondland, Mr Trump said, "I hardly know the gentleman". If the president was taking direct calls from the ambassador after Ukrainian meetings, however, that assertion seems less believable.

Later on Wednesday, during a joint press conference with Turkish President Recep Erdogan, the president went on record saying he didn't recall the call - "not even a little bit".

"I know nothing about that," he said. "First time I've heard it."

That sets up the possibility that Holmes or Sondland may offer testimony that contradicts the president in the days ahead.

In the meantime - either by happenstance or planning - the first day of open public impeachment hearings has its catchy headline and created a new avenue for investigation - and political speculation.

"Democrats land damning new evidence in impeachment inquiry," heralded the website Politico.

"William Taylor's big impeachment reveal," read The Atlantic.

"Bill Taylor dropped a bombshell," echoed Vox.

Media playback is unsupported on your device Media caption What US coal country makes of Trump impeachment

It was enough to have some Republicans crying foul.

"He prepped for hours to come up here. And then all of a sudden, voila, he gets this miraculous intervention from one of his staffers that reminds him of something?" Republican Congressman Mark Meadows told reporters outside the committee hearing room.

"When we start to look at the facts, everybody has their impression of what truth is."

After the day's biggest revelation, Democrats have reason to be pleased, while the president's team has a new set of headaches.