Remembering 9/11: 'You will always be my hero'

NEW YORK�? Ten years after the 9/11 terror attacks, President Barack Obama was due to mark the anniversary by honoring the dead and urging Americans to recapture the sense of unity kindled then but mostly long since extinguished.

Obama, a state senator from Illinois at the time of the attacks, must now help lead the nation into a new phase of healing. He has called on Americans this weekend to remember and serve, and to come together toward a joint future.

"A decade after 9/11, it's clear for all the world to see ? the terrorists who attacked us that September morning are no match for the character of our people, the resilience of our nation, or the endurance of our values," the president said Saturday in his weekly radio and Internet address.

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The president was expected to repeat that theme Sunday in evening remarks at a Kennedy Center memorial concert, after spending the day visiting all three sites where terrorists crashed planes a decade ago, killing nearly 3,000 people.

Accompanied by first lady Michelle Obama, the president was beginning the day in New York City to take part in the memorial service at ground zero along with former President George W. Bush, among others.

See the memorial to Trade Center victims

The ground zero ceremony was set to include moments of silence marking when planes hit the twin towers as well as when they collapsed. Other moments of silence will mark when a plane hit the Pentagon and another crashed in Shanksville, Pa., after passengers fought back against the hijackers. It was believed the hijackers intended to fly the jet into the White House or the Capitol.

From there the president was due to travel to Shanksville.

Then Obama was set to return to Washington to lay a wreath at the Pentagon and attend the "Concert for Hope" at the Kennedy Center, a ceremony of music and readings intended to offer a sense of renewal.

See the memorial to the Pentagon victims

On Saturday, the president stopped at Arlington National Cemetery to visit graves of troops killed in Iraq and Afghanistan , the two long wars he inherited and is beginning to wind down. He also spent time with his family working at a soup kitchen, and called on other Americans also to participate in a day of service.

Throughout the day, the president and his national security team tracked a tip about a possible attack being planned by al-Qaida for New York or Washington to coincide with the anniversary , but U.S. intelligence did not find evidence that terrorists had been sneaked into the country to carry out such a strike.

Video: Solemn day of solidarity in NYC (on this page)

Law enforcement authorities were on high alert and security was especially tight in Manhattan, where police set up vehicle checks on city streets as well as bridges and tunnels coming into the city.

Bush, who has kept a low profile since leaving office, was in Shanksville on Saturday . "The memory of that morning is fresh, and so is the pain," Bush told a crowd at the site.

See the memorial to the Flight 93 victims

New Jersey Governor Chris Christie spoke Saturday at the opening of a monument to the 746 residents of his state killed in the attacks. The "Empty Sky" memorial in Liberty State Park, across the Hudson River from the World Trade Center, has the names of the dead etched on two 30-foot tall walls, each 208 feet and 10 inches long ? the exact width of the twin towers.

"Their lives mattered," Christie said at the ceremony, which began late because security slowed traffic. "That's why we built this memorial and that's why we come here today."

Video: Preserving stories, voices of 9/11 forever (on this page)

Security concerns were high in Washington, too. Authorities shut down part of Dulles International Airport in northern Virginia Saturday because of a suspicious object but later said no explosives were found.

A decade later, after a faltering start, there are signs of rebuilding progress at the World Trade Center site. The new One World Trade Center skyscraper rises more than 80 stories above the ground as it inches to its planned 1,776 foot height ? symbolic of the year of America's independence from Britain.

The Associated Press and Reuters contributed to this report.

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