President Bush wrapped up a day of activities on this grim anniversary of the Sept. 11 attacks by laying a wreath at Ground Zero.

He and First Lady Laura Bush also greeted family members of some of the nearly 3,000 people who died at the World Trade Center last year.

It was his last stop to the three sites where the terror attacks occurred one year ago.

In Shanksville, Penn., where passengers on American Flight 93 fought hijackers to take down a plane believed to be headed to Washington, D.C. -- either to the Capitol or White House, both of which had been evacuated one year ago -- Bush put on his glasses and reviewed a paper in his hand, apparently to stop himself from crying during a chorale performance of the "Battle Hymn of the Republic."

This was the president's first visit to the Pennsylvania site. His tribute included the laying of a wreath and the embrace of those left behind. He has, in his every speech for months, honored the Flight 93 heroism as "the most vivid and sad symbol" of the kind of American solidarity that was reborn in the attacks.

Earlier, during a Pentagon ceremony marking the attack that killed 184 passengers and military personnel, Bush renewed the U.S. commitment to winning the war against terror.

"The murder of innocents cannot be explained, only endured. And though they died in tragedy, they did not die in vain," Bush said outside the new face of the western wall of the Pentagon.  "Their loss has moved a nation to action in a cause to defend other innocent lives across the world. This war is waged on many fronts. We've captured more than 2,000 terrorists. A larger number of killers have met their end in combat."

"This nation honors all who died in our cause," he added.

The president and first lady observed a moment of silence on the South Lawn of the White House at 8:46 a.m. this morning, the time that the first plane hit the World Trade Center one year ago. That followed their attendance at a service at St. John's Church, across Lafayette Park from the White House.

And while the president did not make a public address at that time, in New York, Gov. George Pataki read the Gettysburg Address. The ceremony was marked by a recitation of the names of the 2,801 people killed at the World Trade Center.

The first official observance of the day America changed came at midnight, with the lowering to half-staff of the American flag that flies over the White House.

The act, of course, commemorates the hijacking of four commercial airliners that were used as missiles to hit the World Trade Center, the Pentagon and a Pennsylvania field.

Bush was scheduled to give a prime-time address to the nation featuring Ellis Island's Statue of Liberty as a backdrop.

In New York, residents awoke to words of consolation by the president, who wrote in a New York Times opinion piece that "the terrible illumination of these events has also brought new clarity to America's role in the world."

On Tuesday, that clarity amounted to closing of several U.S. embassies in the Middle East, Africa and Asia and a heightened state of alert in the United States following new intelligence that warned of car bombings, suicide attacks and other anniversary-linked strikes.

The change in alert status marked the first such escalation since the system was created in the wake of last year's attacks.

On Tuesday, Bush said one reason behind the move was that intelligence officers have picked up on threats that sounded very similar to what they were hearing one year ago at this time.

As visible as Bush is being on this Sept. 11, Vice President Cheney was entirely unseen.  He has been at a secure, undisclosed location since Monday night, making brief appearances back in Washington, but primarily tucked away to preserve the presidential line of succession.

He did not appear at the White House ceremony, and was not expected to be seen throughout the day.

At home, the president gripped first lady Laura Bush's hand as they attended service at the yellow-steepled church across from the White House.

"Sept. 11 is a day we are never going to forget -- are we?" the Rev. Luis Leon asked the congregation. "But it did not break us. They have bloodied us but they did not break us." Bush leaned forward and slowly nodded his head in agreement.

Bush told CBS' 60 Minutes II in an interview scheduled to air Wednesday night that flying aboard Air Force One, which circled around Florida, Louisiana and Nebraska to avoid an unsafe Washington, D.C., when the Twin Towers collapsed, was a nightmare for him.

"I can remember sitting right here ... thinking about the consequences of what had taken place and realizing it was the defining moment in the history of the United States," Bush said. "I didn't need any legal briefs, I didn't need any consultations -- I knew we were at war."

Since then, however, the president has been more focused than ever, organizing a war against terror that started with the liberation of Afghanistan, developing a Department of Homeland Security and trying to rally support for taking the war to Iraq, where Saddam Hussein is said to be building weapons of mass destruction.

All this, while the president is also trying to quell hostile feelings by many Americans toward Arabs and Muslims in their communities, who, because of their ethnicity and religion, have been viewed by many as the reciprocal target for the attacks.

"In our war against terror, we must never lose sight of the values that makes our country so strong, the values of respect and tolerance, the values that we believe that everybody ought to worship the Almighty, however they so choose," Bush said Tuesday at a speech at the Afghan embassy.

Nonetheless, as the nation's capital is swathed in live anti-aircraft missiles mounted on launchers to protect the city, the president continues to build support for expanding the war on terror, including trying to win over allies, speaking to the United Nations General Assembly on Thursday and addressing military personnel Wednesday at the Pentagon, which was struck by the hijacked American Airlines Flight 77.

At the Pentagon on Wednesday, remembrances and changes prompted by last Sept. 11 were boldly on display.

Hundreds streamed off the subway and were handed small American flags as they headed to a ceremony at the rebuilt section of the five-sided military headquarters where one of the jets crashed into flames, killing 189 people.

Performances by the Navy and Army bands, a fly-over by four F-16's jets and attendance by many of the nearly 100 injured in the attack punctuated the day's events.

Fox News' James Rosen and the Associated Press contributed to this report.