New York: Americans on Sunday remembered the horror of September 11, 2001, and the nearly 3,000 people who died in the hijacked plane attacks as authorities worked to ensure the emotional 10th anniversary was peaceful.
Law enforcement authorities in New York and Washington were on high alert against what was described as a "credible but unconfirmed" threat of an al Qaeda plot to attack the United States again a decade after the toppling of the World Trade Center`s twin towers by hijacked airliners.
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. There was an unprecedented show of force on New York streets, from roadblocks on Times Square in midtown to the area surrounding Ground Zero further to the south.
Security in lower Manhattan included police barricades on every block near the World Trade Center site with police asking people for identification. People gathered near the site, some clutching American flags, to watch a large screen set up to show a remembrance ceremony here. Some wore T-shirts reading, "Never Forget," a slogan popular since the attacks.
"It was our Pearl Harbor," said John McGillicuddy, 33, a teacher from Yonkers, New York, getting coffee and carrying two American flags on his way to the World Trade Center, referring to the Japanese attack that led America to join World War Two.
"Every year, September is always rough," he said, as he prepared to grieve his uncle, Lieutenant Joseph Leavey, a New York firefighter who died in the south tower on September 11.
"Things have gotten better, we are more aware as a country about things going on in the world," he said.
President Barack Obama arrived in New York on Sunday and will be joined by former President George W. Bush, who was president at the time of the attacks, at the ceremony at Ground Zero.
They were set to join victims` families to hear the reading of the names of those who died on September 11. Bells will toll across the city.
Pope Benedict prayed for September 11 victims and appealed to those with grievances to "always reject violence as a solution to problems and resist the temptation to resort to hate."
In the September 11 attacks, 19 men from the Islamic militant group al Qaeda hijacked airliners and crashed them into the World Trade Center, the Pentagon outside Washington and a field in Shanksville, Pennsylvania. Just weeks after the attacks, US forces invaded Afghanistan to topple that country`s Taliban rulers who had harbored the al Qaeda leaders responsible for the September 11 attacks.
In Afghanistan, NATO-led forces said on Sunday that on the eve of the 10th anniversary a suicide bomber driving a truck of firewood attacked a NATO base in central Afghanistan in an operation for which the Taliban later claimed responsibility.
NATO said the Saturday afternoon attack killed two Afghan civilians and injured 77 NATO troops, but did not state the nationality of the troops. It said those injuries were not life-threatening.
To mark the 10th anniversary, Obama was set to visit all three attack sites.
"There should be no doubt: today, America is stronger and al Qaeda is on the path to defeat," Obama said in his weekly radio and Internet address on Saturday.
US forces killed al Qaeda leader Osama bin Laden in Pakistan in May.
Sunday`s Ground Zero ceremony was set to include moments of silence marking when the 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 after passengers fought back against the hijackers.
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.
`Their lives mattered`
New Jersey Governor Chris Christie spoke on 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."
Security concerns were high in Washington, too. Authorities shut down part of Dulles International Airport in northern Virginia outside the US capital on Saturday because of a suspicious object but later said no explosives were found.
New Yorkers, accustomed to heightened security and alerts that have become commonplace over the past decade, appeared to take the increased police presence in stride.
A decade later, after a faltering start, there are signs of rebuilding progress at the World Trade Center. The new One World Trade Center 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 memorial plaza is ready and the neighborhood has enjoyed a revival, making it a trendy Manhattan place to live.
The 2001 attacks were followed by US-led wars in Afghanistan and Iraq, the latter of which Obama opposed. The United States still has thousands of troops deployed in both countries.