Former President George W. Bush spoke about the threat of domestic terrorism on Saturday during a speech to mark 20 years since the September 11, 2001, attacks that occurred during his presidency.
"We have seen growing evidence that the dangers to our country can come, not only across borders, but from violence that gathers within," Bush said. "There is little cultural overlaps between violent extremists abroad and violent extremists at home ... they are children of the same foul spirit, and it is our continuing duty to confront them."
Bush also called for unity amid growing political division in the US.
"When it comes to unity of America, those days seem distant from our own," he said. "Malign force seems at work in our common life ... so much of our politics has become a naked appeal to anger, fear and resentment."
Bush and his wife, Laura Bush, appeared at the 9/11 memorial site in Shanksville, Pennsylvania, where United Airlines Flight 93 crashed after passengers overcame al Qaeda militants who had hijacked the aircraft.
The 9/11 attacks, which killed 3,000 people in New York, Washington and Shanksville, prompted Bush to launch a US-led invasion of Afghanistan that ousted the Taliban from control in Kabul and sent al Qaeda leader Osama bin Laden into hiding.
His administration's subsequent invasion of Iraq on the erroneous claim that Saddam Hussein's authoritarian government had illicit weapons of mass destruction diverted resources and attention from Afghanistan, leaving U.S. strategy there adrift, former officials and experts say.
Biden's withdrawal of remaining US military forces in Afghanistan at the end of August, months after a deadline set by his predecessor Donald Trump, triggered harsh criticism from both Democrats and Republicans, as the lightning-fast Taliban takeover stranded Americans and Afghans seeking to evacuate.
In a July interview with German state broadcaster Deutsche Welle, Bush called the pullout a mistake and said he worried "the consequences are going to be unbelievably bad."
Speaking about U.S. veterans who served in Afghanistan, Bush said "you have been a force for good in the world and nothing that has followed can tarnish your honor."