President Trump and the first lady, Melania Trump, led a moment of silence at the White House before going to the Pentagon, where 64 people aboard a hijacked American Airlines jet were killed, along with 125 people in the building. The president said that any terrorist who comes to the United States would be met with a force “the likes of which the United States has never used before.”

Mr. Trump delivered his remarks at the Pentagon days after canceling peace talks with the Taliban, which ruled Afghanistan in 2001 and provided a haven for Al Qaeda, the terrorist group that hijacked the planes in the attacks. In Shanksville, Vice President Mike Pence spoke at an observance celebrating the heroism of the passengers aboard the plane who took on the hijackers and sacrificed their lives.

At ground zero, readers began reciting the names of the dead, one by one — brothers, sisters, cousins, mothers, husbands, wives, a solemn process that lasted nearly until the end of the ceremony, shortly after noon.

Some family members brushed away tears as the names were read. Some carried flowers or wore T-shirts with names. Some held placards above the crowd with images of their loved ones. And others who attended said they had their own traditions that they followed.