The names of the dead will be read out in a remembrance service at Ground Zero in NY on the site of the rebuilt World Trade Centre, and President Barack Obama will address a ceremony at the Pentagon.
Americans remembered the victims of the September 11, 2001 attacks on Sunday at a ceremony marking 15 years, with the recital of their names, tolling church bells and a tribute in lights at the site where New York City's massive twin towers collapsed.
The ceremony is expected to include six.
The planes - United Flight 93 from New Jersey, American Flight 77 from Washington, and United Flight 175 and American Flight 11 from Boston - were selected because they were all cross-country flights and carried enough fuel to cause maximum damage. Osama bin Laden, mastermind of the 9/11 attacks, was killed in 2011 by USA special forces, but there are many others who have chosen to follow in his bloody footsteps and wage a terrorist war on America.
The second tower of the World Trade Center bursts into flames after being hit by a hijacked airplane in NY in this September 11, 2001 file photograph.
Some people had left individual white roses and mini American flags on the fountains' black granite edges, where the names of victims are engraved.
The reading of the names ceremony will be interrupted several times by the church bells. The people who died on September 11 weren't expecting it to be their last day. According to a news release, the league will play pre-recorded video messages by Obama and former President George W. Bush at all its Sunday games.
Tom Acquarviva's 29-year-old son Paul was one of 658 employees of financial services firm Canter Fitzgerald who perished after the first plane struck the north tower just below where they worked on the 101st to 105th floors.
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Relatives of victims and senior level USA government officials will be taking part in the ceremony.
"Not a day goes by that we don't remember him", Mr Acquarviva said. Numerous first responders died while running up stairs in the hope of reaching victims trapped on the towers' higher floors.
No public officials spoke at the NY ceremony, in keeping with a tradition that began in 2012.
At about 8:46 a.m. (EST), American Flight 11 crashed into the North Twin Tower.
At the World Trade Center site on Friday, workers constructed the stage for Sunday's ceremony as tourists crowded the memorial fountains built into the footprints of the two original towers.
As evening falls across New York City on Sunday, spotlights will project two giant beams of light into the night sky to represent the fallen twin towers, fading away at dawn.
Countries in the Middle East from Libya to Syria are engulfed in war, providing a fertile ground for Al-Qaeda affiliates to breed, and Europe has been tormented by attacks inspired by the Islamic State group.