Former national security adviser John BoltonJohn BoltonThe Hill's 12:30 Report - Speculation over Biden's running mate announcement Ex-Trump adviser, impeachment witness Fiona Hill gets book deal Hannity's first book in 10 years debuts at No. 1 on Amazon MORE at a private event on Thursday defended the officials who testified in the House impeachment inquiry into President TrumpDonald John TrumpNew Bob Woodward book will include details of 25 personal letters between Trump and Kim Jong Un On The Money: Pelosi, Mnuchin talk but make no progress on ending stalemate | Trump grabs 'third rail' of politics with payroll tax pause | Trump uses racist tropes to pitch fair housing repeal to 'suburban housewife' Biden commemorates anniversary of Charlottesville 'Unite the Right' rally: 'We are in a battle for the soul of our nation' 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 VindmanVindman describes 'campaign of bullying, intimidation and retaliation' by Trump, allies in op-ed Vindman marks 1 year since call that led to Trump's impeachment White House officials alleged Vindman created hostile work environment after impeachment testimony: report MORE, Bill Taylor, the former top U.S. diplomat to Ukraine and former Ambassador to Ukraine Marie YovanovitchMarie YovanovitchMarie Yovanovitch on Vindman retirement: He 'deserved better than this. Our country deserved better than this' Cheney clashes with Trump Voters must strongly reject the president's abuses by voting him out this November 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.