WASHINGTON (AP) — The ellipses have long baffled lawmakers: What did President Donald Trump say on his call with Ukrainian President Volodymr Zelenskiy that the White House may have omitted on the rough transcript released to the public?

Lt. Col. Alexander Vindman began to fill in those gaps.

In testimony for the House impeachment inquiry , Vindman, who worked at the White House National Security Council and monitored the July 25 phone call, told investigators he heard a discussion of Biden and Burisma — a reference to the gas company where Joe Biden's son served on the board — and that some of the ellipses omitted Trump saying there are recordings of Joe Biden discussing corruption in Ukraine, according to people familiar with Tuesday's closed-door testimony.

Vindman testified to House investigators that he tried to add the missing words to the transcript, but was unsuccessful, the people said.

The striking development provides new insight into the phone conversation that set off the impeachment inquiry and the extent to which the White House sought to shield the president's actions and words from the public.

"As they said during Watergate, it's the cover-up," said Rep. Mark DeSaulnier, a California Democrat who sits on the House Oversight Committee. "There was clearly an attempt at the White House to cover up."

The House impeachment inquiry is looking into Trump's call, in which he asked Zelenskiy for a "favor" — to investigate Democrats in the 2016 election and Biden, a potential 2020 rival — as the Trump administration held up military aid for the Eastern European ally confronting Russia.

It is illegal to seek or receive assistance from a foreign entity in U.S. elections, and Democrats say this was a quid pro quo for political gain and an impeachable offense.

A government whistleblower claimed there was an attempt at the White House to cover up Trump's call by moving the transcript onto a secure server accessible to fewer people in the government.

The White House ultimately released the rough transcript after the whistleblower complaint, but insists Trump did nothing wrong. Trump's team has said the president was concerned about rooting out corruption in Ukraine.

In the White House rough transcript, there are three parts of the telephone call where ellipses replace what was said.

In one, Trump asks Ukraine's president, "I would like you to find out what happened with this whole situation with Ukraine, they say Crowdstrike..."

That's a reference to the 2016 election, when Democrats hired the cyber firm, which determined Russia had hacked the party's email. Trump is airing a discredited conspiracy theory that CrowdStrike may have had ties to Ukraine and the hack was a setup designed to cast blame on Russia.

"I guess you have one of your wealthy people...," Trump says in another.

Later Trump said, "Biden went around bragging that he stopped the prosecution so if you can look into it ..."

Some of the ellipses omitted Trump's suggestion to Zelenskiy that there are recordings of Joe Biden, according to one of the people familiar with the testimony.

It is unclear what recordings Trump was referring to on the call. But the president and his personal lawyer, Rudy Giuliani, have defended their actions by citing a video of Biden from 2018. Speaking on a public panel, Biden recounted threatening to withhold a $1 billion loan guarantee from Ukraine's government unless it fired a prosecutor that the U.S. and other countries said was failing to fight corruption. Ukraine fired the prosecutor.

Vindman told House investigators his efforts to add back the missing language were unsuccessful and the White House released the transcript with the gaps, the people said.

There is no tape recording of the July 25 call . The White House stopped taping presidential calls in the 1970s when President Richard Nixon recorded 3,700 hours of conversations, transcripts of which were used by Watergate investigators and during impeachment hearings that followed.

Instead, various officials listen into presidential calls and take extensive, real-time notes that are then turned into a rough transcript.

For this call, Vindman was among those convened in the Situation Room and elsewhere to monitor the call. He testified that he listened with colleagues from the NSC and Vice President Mike Pence's office.

Vindman later reported what he heard to the NSC's lead counsel. It was the second time he had reported his concerns about the Trump administration's actions toward Ukraine that are now central to the impeachment inquiry.

"I realized that if Ukraine pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained," Vindman wrote in his prepared remarks to the House impeachment investigators. "This would all undermine U.S. national security."