WASHINGTON — President Joe Biden’s nominee to be the Pentagon’s top weapons buyer, Defense Innovation Unit director Mike Brown, withdrew from consideration on Tuesday, Defense News confirmed.

In a letter to Defense Secretary Lloyd Austin obtained by Defense News, Brown said he was withdrawing because an inspector general investigation into hiring practices at DIU was expected to delay consideration of his nomination by up to a year.

”While I am confident the Office of the Inspector General will ultimately find no wrongdoing on my part, I know there are other qualified candidates who can focus on the urgent business of making our acquisition process faster and more cost-effective,” Brown said in the letter. “I must put the interests of the Department above my own enthusiasm for serving as Under Secretary for Acquisitions and Sustainment.”

Pentagon spokesman John Kirby said Austin respects Brown’s decision and thanked him “for his willingness to be nominated for this challenging position.” Austin “remains grateful for Mr. Brown’s continued service at the Defense Innovation Unit,” Kirby said.

News of Brown’s withdrawal was first reported by Inside Defense.

Numerous defense innovation experts expressed dismay over Brown pulling out as nominee. Because of his past career in the tech sector and work at DIU, many had high hopes for Brown’s ability to reform the department’s arcane acquisition practices that doesn’t suit small innovative, nontraditional contractors offering new technologies that primes don’t.

Chris Brose, former Senate Armed Services Committee staff director, said Brown had a “proven track record of rapid acquisition and fielding of advanced technologies” and called his withdrawal a huge setback.

“You need someone who understands what that ecosystem looks like, that understands what the cutting edge of advanced technology is and who’s developing it, what nontraditional defense companies and new entrants are going through in trying to begin and scale their work with the Department of Defense,” said Brose, now chief strategy officer at defense tech firm Anduril Industries. “He, better than anybody, understands all of that.”

As undersecretary of defense for acquisitions and sustainment, Brown would have overseen a budget of more than $100 billion for major defense programs — such as the F-35 fighter jet, aircraft carrier elevator and efforts to speed up software acquisitions. He would have been highly influential over defense industry matters and been responsible for maintaining America’s military edge.

Brown has led the Pentagon’s Silicon Valley outreach arm since 2018 and worked to connect small startups developing innovative technologies with Department of Defense components. Brown’s initial nomination was praised by observers, including several former senior DoD officials, because of his tech industry background and understanding of the challenges nontraditional contractors face working with the DoD.

“One of the exciting aspects of his nomination was the combination of a few different skill sets and knowledge bases that he would have brought to the role,” said Lindsey Sheppard, a fellow at the Center for Strategic and International Studies.

“Whenever you don’t have confirmed leadership in senior positions at the Department of Defense, you’re losing,” Brose said. “You don’t have people with the mandate and the legitimacy to take decisions and risky decisions to the fullest extent of the authorities of that office.”

Brown joined government in 2016 as a White House presidential innovation fellow after a long career in the tech sector capped by two years as CEO of cybersecurity software giant Symantec. As a fellow, he wrote a detailed report on the threat the Chinese government posed to the U.S. venture capital system.

Under Brown’s guidance, DIU has transitioned increasing amounts of projects to DoD components. In 2020, DIU transitioned 11 projects, up from four in 2018. One of DIU’s most significant achievements under Brown’s leadership was providing an alternative to Chinese-made small drones under a DIU project called Blue sUAS, which made trusted small drone options available to the whole federal government.

Bill Greenwalt, nonresident senior fellow at the American Enterprise Institute and former deputy undersecretary of defense for industrial policy, called the news “sad and depressing.”

“He had the potential, based on his background, to really bring in and leverage this massively innovative sector of the U.S. and international economy and then fold that into how we can create better capability against China,” Greenwalt said.

Arnold Punaro, a former staff director with the Senate Armed Services Committee and the National Defense Industrial Association board chairman, said he is disappointed to see Brown out of the running.

“The DOD acquisition system needs strong leadership to implement innovation, speed, and cost-effectiveness, and Mike was the perfect choice for the position,” Punaro said in a statement. “Mike’s strong leadership will continue to benefit DIU and the Department moving forward, and we are lucky to have him there.”