Two of the jets brought down the Twin Towers of New York City's World Trade Centre, another extensively damaged the Pentagon outside Washington and a fourth crashed in a field in rural Pennsylvania when passengers aboard that flight revolted against the hijackers.
At Ground Zero in New York where the towers once stood, more than 1000 relatives of those killed and others gathered for the annual reading of the list of 2983 people killed at the three sites. The list excludes the 19 hijackers, who also died.
The reading began at 8:39am local time, with pauses for moments of silence at 8:46am, 9:03am, 9:37am and 10:03am, the time of impact for the four planes, and again at 9:59am and 10:28am, the times that the north and then the south tower fell.
As the moment of the reading approached, family members, uniformed police and firefighters milled about the vast, twin reflecting pools that mark the footprints of the two towers, their edges etched with the names of the victims.
Many carried or wore pictures of their loved ones.
Alyson Low, 41, of Fayetteville, Arkansas, carried a picture of her sister, Sara Elizabeth Low, who was a flight attendant on American Airlines Flight 11, the first plane to crash, striking the trade centre's north tower.
The reading of names began with Patricia Abbott, wife of Alan Jay Richman, who died at the trade centre.
In previous years, politicians including US presidents, governors and New York City mayors have participated in the reading of the names, or have read from the Bible or recited passages from literature.
This year only the families of the more than 2750 who were killed at the trade centre appeared on the podium to read their names.
Politicians still attended, but under event rules set down in July by the National September 11 Memorial and Museum, chaired by New York Mayor Michael Bloomberg, none spoke or participated in the reading of names.
Homeland Security Secretary Janet Napolitano attended the New York ceremony this year.
The restrictions did not extend to politicians at the other remembrances.
President Barack Obama observed a moment of silence for the September 11 victims on the South Lawn of the White House.
Flanked by a flag-bearing military honour guard, the President and First Lady Michelle Obama stood solemnly with heads bowed. They placed their right hands over their hearts while a bugler played Taps.
Some in the crowd of about a hundred had tears in their eyes. The Obamas then turned and, holding hands, walked back into the White House residence.
Vice-President Joe Biden was in attendance to deliver remarks in Shanksville, Pennsylvania, where 40 passengers aboard United Flight 93 were killed when that plane crashed as they fought back against their hijackers.
"How we handle the legacy of these 40 people and what they did, what they kept from happening, is really more of a statement about ourselves, about what we value as a society," said Patrick White, president of the organisation Families of Flight 93.
Mr White's cousin, Joey Nacke II, was among the passengers who stormed the cockpit.
US authorities say the hijackers planned to crash that plane into the US Capitol in Washington.
