Lieutenant General Russel Honoré made the comment during an appearance on MSNBC on Tuesday.
"It's my personal opinion that the executive branch was complicit in the planning and the delayed response that occurred in bringing in more federal assistance to the Capitol that day," he said.
Lt General Honoré was selected by House Majority Leader Nancy Pelosi to lead an inquiry into the security at the US Capitol in the wake of the riot.
He said that Mr Trump's statements – and lack of condemnation – concerning the riot influenced his conclusion.
“'This was not a riot, it meant no harm, it was like a picnic,'” Lt General Honoré said, summarising Mr Trump's statements. “The last I heard from him, he told them to go to the Capitol and raise hell.”
The retired military officer was likely referencing Mr Trump telling his supporters to “fight like hell” during a speech at the White House just before the Capitol riot occurred.
“You'll never take back our country with weakness. You have to show strength, and you have to be strong,” Mr Trump said during the rally. “We fight like hell, and if you don't fight like hell, you're not going to have a country anymore.”
Prior to the rally, Mr Trump called on his supporters to come to Washington DC and attend his rally, using the tag line “be there, will be wild!”
The comments come as the House select committee is in the opening stages of its inquiry into the Capitol riot. On Tuesday, several police officers involved in defending the Capitol from a mob of Trump supporters on 6 January testified about their experiences and the injuries they sustained.
Mr Trump was impeached for a second time on charges that he incited the riot, but the former president was ultimately acquitted thanks to the efforts of Senate Republicans.
Republican media and lawmakers have similarly been trying to offer counter-narratives to distract from the testimonies during the House-led investigation into the Capitol riot.
Fox News hosts Laura Ingraham and Tucker Carlson mocked the testimonies of the officers during their shows, and House Minority Leader Kevin McCarthy has railed against Ms Pelosi for rejecting two Republican lawmakers he wanted included on the panel.
Representatives Matt Gaetz and Marjorie Taylor Greene attempted to hold a press conference immediately after the committee closed for the day. That press conference was ultimately derailed when protesters gathered and accosted the duo with whistles and signs that read "traitors".
More than 540 people have been arrested for participating in the deadly riot, and the FBI hopes to apprehend hundreds more.
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