President Donald Trump has presided over his first 9/11 commemoration in office, marking the 16th anniversary of the attacks, according to the BBC.
He and first lady Melania observed a solemn moment of silence at the White House and later at the Pentagon.
Thousands are gathering nationwide to mark the deadliest attack on US soil.
Nearly 3,000 people died after hijackers crashed planes into New York's World Trade Center, the Pentagon and a rural Pennsylvania field in 2001.
"Our values will endure, our people will thrive, our nation will prevail, and the memory of our loved ones will never, ever die," Mr Trump said outside the Pentagon.
Mr Trump joined Defence Secretary James Mattis and General Joseph Dunford, chairman of the Joint Chiefs of Staff, at the Department of Defense to remember the lives lost on 9/11.
"The terrorists who attacked us thought they could incite fear and weaken the spirit. Those who try will soon join the long list of vanquished enemies who dare to test our mettle," he said.
Earlier, Mr Trump appeared at a White House for a commemoration that took place at the same time relatives of 9/11 victims, survivors, rescuers and others gathered at New York's 9/11 memorial on Monday at 08:46 local time (12:46 GMT) to mark the exact time the first plane struck the World Trade Center's North Tower.