Al Qaeda has been severely weakened in the decade since the 9/11 attacks while the United States has demonstrated a spirit of resilience, Defense Secretary Leon Panetta said on Tuesday.
Speaking in New York City after he paid a visit to the stunning new 9/11 memorial at Ground Zero, Panetta said Americans could take heart that al Qaeda had suffered a series of blows in recent years.
"We can take great pride that since 9/11 we have achieved very significant success in going after al Qaeda, in going after al Qaeda's leadership," he told reporters.
Three of the top four leaders in al Qaeda, including Osama bin Laden who was killed in a controversial US raid in May, are now dead, seriously damaging the organization, Panetta said.
But he warned that America had to remain vigilant and keep relentless pressure on the global terror network blamed for the September 11, 2001 assault that claimed nearly 3,000 lives.
"They continue to plan attacks and I don't think we can take anything for granted," he said.
As CIA director until he took over the Pentagon in July, Panetta presided over an intense drone bombing campaign against militants in Pakistan as well as the raid by US Navy commandos that killed bin Laden.
Launching a week of events marking the 10th anniversary of 9/11, Panetta took a tour of the new memorial and a museum still under construction, accompanied by troops who enlisted following the attacks.
Rain, however, forced him to cancel plans to head to Shanksville, Pennsylvania, where one of the four planes hijacked a decade ago crashed into a field, after passengers overpowered the hijackers.
Amid a steady downpour, New York City Mayor Michael Bloomberg escorted Panetta around the memorial site, walking past rows of white oak trees to two giant, sunken fountains that rest in the footprints of the old Twin Towers.
The granite fountains evoke somber waterfalls, with rushing water flowing 30 feet down to a central square below, conveying the terrible losses suffered on September 11.
The names of the victims are carved into a dark parapet on the edges of the enormous fountains, with new skyscrapers now rising nearby.
A few steps from one of the fountains, Panetta stood before a callery pear tree that has become a symbol of survival.
The tree was found charred in the rubble after the attacks, but was brought back to health at a nursery in the Bronx and replanted at Ground Zero.
Later, Panetta said he was moved by the memorial, describing it as "a special place."
Even as Americans mourn those killed on 9/11, Panetta said it was also a time to reflect on the determination shown by the country in the years since.
"You come back and see the changes that have happened here, and you know it's a sign that this country truly is resilient."
Panetta brought along a member of each branch of the armed forces -- the Army, Navy, Marines, Air Force and Coast Guard -- who signed up to serve in uniform in the aftermath of the attacks, officials said.
Four of the five troops traveling with Panetta had deployed to Iraq or Afghanistan since 9/11, including Air Force Staff Sergeant Robert Gutierrez, 31, who directed air strikes for Army special forces in Afghanistan.
The forward air controller was wounded on one of his tours and has been awarded the prestigious Bronze Star for valor.
Officials said Panetta is the first cabinet member to get a first-hand look at the 9/11 memorial in New York, which is due to be formally unveiled Sunday, the 10th anniversary of the attacks.
Later this week, the defense chief will attend events in remembrance of those killed at the Pentagon on 9/11, when a hijacked plane struck the Defense Department headquarters shortly after two other hijacked airliners slammed into the Twin Towers in Manhattan.
The September 11 attacks killed 2,977 people. The vast majority, 2,753, died in New York, while 184 people perished at the Pentagon and another 40 at Shanksville. Those figures exclude the 19 hijackers.