In New York, family members have finished reading the names of nearly 3,000 victims killed in 2001 in the worst terror attack in U.S. history. Family members asked the public not to forget their loved ones and the attack, which caused the World Trade Center's collapse and sent hijacked planes into the Pentagon and a Pennsylvania field.
But some cities either scaled back or canceled their ceremonies and fewer people attended them.
Politicians did not speak at the New York ceremony for the first time.
President Barack Obama and first lady Michelle Obama laid a wreath at the memorial to Pentagon victims. Vice President Joe Biden spoke in Shanksville, Pa.