Co-chairs of the committee investigating the January 6 Capitol riot are demanding Pentagon records related to recent reports that Chairman of the Joint Chiefs of Staff Mark Milley went behind then-President Donald Trump 's back to warn China of a potential attack. 'The facts surrounding steps taken at the Pentagon to protect our security both before and after January 6th are a crucial area of focus,' Representatives Bennie Thompson, a Democrat, and Liz Cheney, a Republican, wrote in a statement Thursday. 'Indeed, the Select Committee has sought records specifically related to these matters and we expect the Department of Defense to cooperate fully with our probe,' the two lawmakers added. 'Looking ahead, we will carefully evaluate all the facts.' Excerpts from upcoming book 'Peril', which were published earlier this week, detail that Milley sought to assure in calls with General Li Zuocheng of the People's Liberation Army in China that the U.S. was not about to launch strikes in the aftermath of the Capitol attack. The select committee probing the January 6 Capitol riot are demanding the Pentagon turn over all documents related to claims that Chairman of the Joint Chiefs of Staff Mark Milley spoke to China behind then-President Donald Trump's back In one call he said he vowed to circumvent Trump and use backchannels to alert his counterpart if an attack were imminent. 'Things may look unsteady,' Milley told Li. 'But that's the nature of democracy, General Li. We are 100 percent steady. Everything's fine. But democracy can be sloppy sometimes.' 'General Li, you and I have known each other for now five years. If we're going to attack, I'm going to call you ahead of time. It's not going to be a surprise,' Milley reportedly told him. Revelations from the book, authored by journalists Bob Woodward and Robert Costa, immediately led to calls from retired military leaders for Milley to resign or even face court masahl. Trump – and some of these retired military leaders – claim that if the report is true, then Milley committed 'treason.' 'If this is true GEN Milley must resign,' retired Lieutenant Colonel Alexander Vindman tweeted on Tuesday along with an article about new book that claims Milley was fearful Trump might spark war with China in the wake of his November election defeat to Joe Biden. Vindman added: 'He usurped civilian authority, broke Chain of Command, and violated the sacrosanct principle of civilian control over the military. It's an extremely dangerous precedent. You can't simply walk away from that.' Vindman was the star witness in the first impeachment of President Trump. 'Peril' is set to hit shelves on September 21. Milley's spokesman Colonel Dave Butler essentially confirmed the call, saying on Wednesday: 'The Chairman of the Joint Chiefs regularly communicates with Chiefs of Defense across the world, including with China and Russia.' 'His calls with the Chinese and others in October and January were in keeping with these duties and responsibilities conveying reassurance in order to maintain strategic stability,' Butler wrote. Retired Lieutenant Colonel Buzz Patterson, an aide during Bill Clinton's administration who protected the 'nuclear football,' said it was not Milley's job to make decisions in a civilian-controlled military. 'As a former carrier of the nuclear football, this is simply unconscionable,' Patterson tweeted. 'The military doesn't get to make this call in a civilian controlled system. Milley needs to resign immediately.' In a second tweet he wrote: I'm outraged and every veteran and active duty member should be too! Each and every one of us were educated and drilled to always respect the chain. The CJCS violated that and usurped the authority of the commander in chief for political purposes. He jeopardized our security.'