Nathan Sales, the former ambassador-at-large for counterterrorism under former President Donald Trump, spoke out on Thursday to reveal what he feels was Joe Biden’s major “most important misstep” in Afghanistan.

Sales told Fox News that Biden’s biggest mistake was leaving the Bagram Air Base, which is more remote and located in an area that is better to aid evacuation efforts.

“One of the most important and mistaken decisions that President Biden made was to withdraw from Bagram Air Base. Bagram Air Base is 30 or so miles north of the capital,” Sales said. “It was a military facility. It was indeed the heart of American counterterrorism efforts in Afghanistan for years. It would have been a far better alternative for carrying out the evacuation that is now going on.”

Sales went on to explain that the would have been “far better” for American evacuation efforts because it is in a remote location that is easier to defend and control. In contrast, Kabul Airport is in a more dangerous area that is easier to attack, as it is in the heart of a busy urban area surrounded by buildings and crowds.

“I think that was probably one of the most important missteps that the White House took over the course of the evacuation,” said Sales, who is a Vandenberg Coalition Advisory Board member.

Sales also said that the U.S. should have extended the perimeter around the airport, and that Biden has never given a good reason as to why this did not happen.

“The United States should not be outsourcing security at the airport to the Taliban,” the counterterrorism expert said. “American lives are at stake and there is no substitute to extending the security perimeter to keep our people safe.”

Sales later said that the attacks that took place near the Kabul airport on Thursday were likely carried out by ISIS Khorasan Province (ISIS-K), which is a more local branch of the global Islamic State of Iraq (ISIS) network. He feels that terrorist threats in Afghanistan will only get worse from here.

“It is very worrying that ISIS-K, if it is indeed them, has been able to carry out a sophisticated, mass casualty attack like this because it suggests that terrorist threat within Afghanistan is going to continue to grow,” said Sales. “And it’s significant because the Taliban and ISIS have been fighting against each other for years.”

“If it was them, the fact that ISIS was able to carry out a sophisticated attack like this, despite Taliban pressure against them, really suggests that Afghanistan going forward is going to be a petri dish for all kinds of terrorist threats,” he continued.

Sales ominously pointed out that the only reason that the U.S. was even able to warn Americans about this attack was that troops were on the ground in Afghanistan collecting intelligence.

“The moment the last C-17 takes off, and we leave the country behind with no military or diplomatic presence on the ground, our ability to collect information about plots like the one that unfolded today is going to be dramatically reduced,” he said.

Sales concluded by calling on Biden to quickly find a way to evacuate the roughly 1,000 Americans that remain in Afghanistan, since the Kabul Airport is clearly not sufficiently secure.