President Barack Obama lays a wreath at the Pentagon Memorial, in Washington, the United States, Sept. 11, 2011, with First Lady Michelle Obama, Defense Secretary Leon E. Panetta and Navy Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, to honor the 184 victims killed when American Airlines Flight 77 crashed into the Pentagon during a terrorist attack 10 years ago. [Xinhua/DOD/Chad J. McNeeley]

 Silent tears and choked-back sobs marked the 10th anniversary on Sunday of the 9/11 attacks on the United States.

The decade was marked across the nation with emotional memorial services in communities large and small to commemorate the victims of one of the country's darkest days.

The largest event, and, of course, the focus of the world's spotlight, was in New York City where the memorial services began in bright and sunny weather reminiscent of that fateful late- summer day 10 years ago that was crystal clear. It turned cloudy as the names of the victims were read aloud.

Nearly 3,000 people were killed on Sept. 11, 2001 when al-Qaeda terrorists hurtled four hijacked jet airliners laden with hundreds of passengers and thousand of kilos of fuel into the twin towers of the World Trade Center (WTC) in New York City's lower Manhattan, the U.S. Defense Department's Pentagon headquarters in the U.S. state of Virginia, across from Washington, DC, and into a field in the U.S. State of Pennsylvania.

The hijackers aboard United Airlines flight 93 apparently were attempting to crash into the dome of the U.S. Capitol when they were thwarted in their attempts by passengers, alerted by cell phones to earlier hijack-crashes, who rushed the militants forcing the jetliner to plummet into farmland near Shanksville, Pennsylvania, and explode in a fireball, killing all aboard.

Also remembered were the six people killed in the 1993 small- truck bombing of one of the twin towers, a failed al-Qaeda attempt to topple the tower.

This year's memorials in the political and financial capitals of the United States were marked in the shadow of intelligence reports of a "credible threat" -- although admittedly uncorroborated -- of a possible truck-bomb attempt in either city. Security had been upped before the ceremonies, then when the new intelligence was reported it was hiked another notch.

There was fear in Washington and in New York for numerous bridges and tunnels where law enforcement personnel stopped vehicles and even inspected backpacks people carried into subways.

Perhaps in reflection of heightened security, U.S. Air Force F- 16 fighter jets scrambled to escort Los Angeles to New York American Airlines flight 34 which reported three passengers locked themselves in the bathroom.

Law enforcement officials said later they did not think the incident was related to terrorism.

U.S. President Barack Obama attended all three of the major memorials, in New York, Shanksville and Washington.