Former national security adviser John BoltonJohn Bolton Trump ignores science at our peril Bolton defends decision to shutter NSC pandemic office US retaliates with missile strikes in Iraq MORE at a private event on Thursday defended the officials who testified in the House impeachment inquiry into President TrumpDonald John TrumpHealth insurers Cigna, Humana waive out-of-pocket costs for coronavirus treatment Puerto Rico needs more federal help to combat COVID-19 Fauci says April 30 extension is 'a wise and prudent decision' MORE

Bolton, speaking in Austin, Texas, said members of the Trump administration should “feel they’re able to speak their minds without retribution,” KXAN reported.

The former national security adviser defended former National Security Council Senior Director for Europe and Russia Fiona Hill, former top National Security Council aide Tim Morrison, Lt. Col. Alexander VindmanAlexander VindmanAmerica's diplomats deserve our respect White House withdraws nomination for Pentagon budget chief who questioned Ukraine aid hold Juan Williams: Will the GOP ever curb Trump? MORE, Bill Taylor, the former top U.S. diplomat to Ukraine and former Ambassador to Ukraine Marie YovanovitchMarie YovanovitchAmerica's diplomats deserve our respect House panel says key witness isn't cooperating in probe into Yovanovitch surveillance President Trump's assault on checks and balances: Five acts in four weeks MORE, the outlet reported.

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The five officials all gave public and private testimony in the House impeachment inquiry into the president.

“All of them acted in the best interest of the country as they saw it, and consistent to what they thought our policies were,” Bolton said during a question-and-answer time after his keynote speech.

“The idea that somehow testifying to what you think is true is destructive to the system of government we have, I think is very nearly the reverse, the exact reverse of the truth,” Bolton added. 

Bolton was in Austin speaking at a private client luncheon at the invitation of Luther King Management, a Texas investment management company, KXAN reported. The program was listed as “Foreign Challenges Facing the Trump Administration.”

Bolton also reportedly mentioned his forthcoming book, “The Room Where It Happened: A White House Memoir” at the event. This week, the White House sent a letter to Bolton’s lawyer seeking to block publication of certain parts of his book, claiming that the former official’s manuscript “contained significant amounts of classified information.” 

The New York Times reported that the book alleges that Trump did not want to release nearly $400 million in security aid to Ukraine until the country announced investigations into the president’s political rivals.

Democrats and some Republican lawmakers in the Senate impeachment trial have demanded to include additional witnesses and documents in the trial, including potentially calling Bolton to testify before the Senate. However, GOP leaders expect to have the votes Friday to defeat the motion.