An officer of the National Guard is expected to invoke whistleblower protection at Tuesday's House hearing probing the use of force during a protest in Washington, D.C.'s Lafayette Square last month, pitting him against President Trump and the acting chief of the U.S. Park Police, who have polar opposite accounts of the same incident.

National Guard Maj. Adam DeMarco said the warnings by officers – required by law – that the park was going to be cleared of hundreds of demonstrators, who he says we largely peaceful, were “barely audible” from his position 20 yards away, according to his pre-written testimony to House Natural Resources Committee.

“From what I could observe, the demonstrators were behaving peacefully,” when Park Police, the Secret Service and other unidentified forces, some mounted on horses, began using clubs and chemical agents to disperse the crowd, as well as other tactics such as punching and beating protesters, DeMarco wrote in his testimony.

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DeMarco is expected to invoke the Military Whistleblower Protection Act, which in part says that no one can block a member of the armed forces from lawful communications with Congress.

The incident, which occurred in the midst of ongoing protests decrying the death of George Floyd while in police custody, garnered widespread attention both because of the aggressive tactics used to clear the park as well as the subsequent appearance of President Trump and other administration officials who walked through the area unannounced to stage a photo shoot in front of the iconic St. John's Church.

In this Monday, June 1, 2020, file photo President Donald Trump walks past police in Lafayette Park after visiting outside St. John's Church across from the White House in Washington. 

Protestesters had for days taken to the park in front of the White House to denounce the use of force by police, particularly targeting Black and Hispanic people, after Floyd, an unarmed Black man, died in police custody after a Minneapolis police officer kneeled on his neck for over eight minutes.

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Trump has denied that the area was cleared merely for a photo op and instead said officers did so to make way for a fence they planned to build with the purpose of blocking protesters from the square.

Meanwhile, U.S. Park Police acting Chief Gregory Monahan said Tuesday in front of House members that the protest activity leading up to the incident at Lafayette Square on June 1 "was one of the most violent protests I've been a part of in my 23 years" of service."

“On the whole, the United States Park Police acted with tremendous restraint in the face of severe violence from a large group of bad actors who caused 50 of my officers to seek medical attention,” Monahan wrote in his prepared remarks. "Our actions as an agency on June 1 centered around public safety and the safety of my officers."

Acting U.S. Park Police Chief Gregory T. Monahan, testifies before a House Natural Resources Committee hearing on actions taken on June 1, 2020 at Lafayette Square, Tuesday, July 28, 2020 on Capitol Hill in Washington. (Bill Clark/Pool via AP)

DeMarco says the events that unfolded happened with little warning for both protesters and officers on the scene after Attorney General William Barr, accompanied by the chairman of the Joint Chiefs of Staff Gen. Mark Milley, spoke with Park Police officials.

DeMarco says he spoke to Milley just moments before officers executed plans to clear the park.

“As the senior National Guard officer on the scene at the time, I gave Gen. Milley a quick briefing on our mission and the current situation,” DeMarco wrote. “Gen. Milley told me to ensure that National Guard personnel remained calm, adding that we were there to respect the demonstrators’ First Amendment rights.”

Meanwhile, DeMarco says he was also told by a Park Police liaison officer that the agency was not using tear gas, and it was only "stage smoke." However, DeMarco said judging from the reaction caused by the smoke – stinging in his nose and eyes – he thought it was tear gas. He said he also found tear gas canisters in the streets later that night.

The violent clearing of demonstrators from the nation's premier protest space in front of the White House on June 1 is spotlighting a tiny federal watch force created by George Washington. Democratic lawmakers want answers about the clubbing, punching and other force deployed by some Park Police in routing protesters from the front of the White House on Monday.

Lawmakers are interested in figuring out who specifically directed the park to be cleared on that day.

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Separately, several other investigations stemming from issues involving the use of force by officers across the nation as protests continue are ramping up on Tuesday as well.

The Justice Department inspector general has launched an investigation into the use of force by federal officers in Portland, Ore., and Washington, D.C., who were deployed to those cities in an effort to protect federal property during demonstrations – as some were vandalized, set on fire or looted in recent weeks.

Meanwhile, city officials have accused officers of concealing their identities and making arrests without giving an adequate reason, as well as using excessive force to detain demonstrators.

Barr himself will face questioning from the House Judiciary Committee as well on Tuesday, about policing tactics and diversity issues in the nation's departments.

The Associated Press contributed to this report.