The officer says “excessive” force was aimed at peaceful protesters.

An officer of the National Guard will appear before Congress on Tuesday to challenge the Trump administration’s story behind the decision to force protesters out of a park near the White House last month.

Adam DeMarco, an Iraq war veteran and major in the D.C. National Guard, was responsible for coordinating the guard’s support of U.S. Park Police during a June 1 protest against police brutality at Lafayette Park.

In testimony submitted to Congress ahead of the hearing, DeMarco described the demonstrators as largely peaceful and said they faced an “excessive use of force” at the hands of federal police the guardsmen were tasked to support.

“Having served in a combat zone, and understanding how to assess threat environments, at no time did I feel threatened by the protestors or assess them to be violent,” DeMarco wrote in his testimony to the House Natural Resources Committee, which conducts oversight of the National Park Service and Park Police.

DeMarco said he could hardly hear the warnings for people to leave the park from his position north of the White House, and said there was “no indication” protesters knew they were required to leave.

Attorney General William Barr orchestrated the clearing of the park just before President Donald Trump walked through the area for a photo op at a nearby church, as ABC News previously reported. Barr later said expanding the security perimeter was already planned when he found out the president intended to walk over.

The administration has denied that tear gas was used on the demonstrators. But DeMarco directly refuted the claim in his prepared testimony released Monday, saying that the irritation in his eyes and nose reminded him of the gas he was trained by the military to detect. He later found spent tear gas canisters on the street near the park.

“Members of the Committee, the events I witnessed at Lafayette Square on the evening of June 1 were deeply disturbing to me, and to fellow National Guardsmen,” DeMarco said.

U.S. Park Police Acting Chief Gregory Monahan is expected to defend the federal law enforcement response before the committee on Tuesday. While he did not mention a specific instance of protester aggression immediately before the move to clear the park, Monahan said acts of violence in the days leading up to June 1 prompted the decision.

“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.

Monahan has maintained that clearing the park at the time was necessary to set up a security barricade around the White House and that moving the protesters out of the park was done “once the fencing arrived.” The claim also appears to contradict DeMarco’s testimony.

The D.C. guardsman said materials to set up the fence did not arrive until 9 p.m. that night — more than two hours after the demonstrators were forced to leave.

Both DeMarco and Monahan face questions before a panel of House lawmakers on Tuesday. Attorney General Barr will also testify in a separate hearing focused on his intervention into high profile criminal cases involving Trump’s political allies.

Their appearances before Congress follow months of protests across the country aimed at calling attention to systemic racism in law enforcement. While the demonstrations have been largely peaceful, some have erupted with violence.

Democrats and civil liberties advocates blame the administration for escalating confrontations between law enforcement and protesters in recent months. Trump has threatened the deployment of additional forces to target crime and protect federal property.

Clashes between demonstrators and federal law enforcement in Portland, Oregon, ignited a weeks-long feud between the administration and local officials with the City Council taking action earlier this month to prevent cooperation between local cops and federal agents.

ABC’s Ben Siegel contributed to this report.