Napolitano said Homeland Security would warn the public if a threat arose.
Asked about the threat of "lone wolves," Napolitano said it is a "key concern" that "more and more Americans in-country are being radicalized to the point of violence" and that Internet recruiting by those like U.S.-born cleric Anwar al-Awlaki is having an impact, CBS News reports.
Speaking generally and not in reference to the 9/11 anniversary, Napolitano said a 9/11-style plot would be "much more difficult" to execute now than 10 years ago because of the multiple security layers now in place, better intelligence gathering and better intelligence sharing, among other improvements, CBS News reports.
In New York, despite no specific terrorism threats against the city, thousands of extra police officers will be deployed, officials said Wednesday.
Police Commissioner Raymond Kelly, in remarks at a security conference in midtown Manhattan, cited evidence found in Osama bin Laden's compound after his death suggesting he hoped to strike on the anniversary.
"For that reason alone, we need to take precautions as if an actual plot is under way," Kelly said.
The New York Police Department plans to form a zone around the World Trade Center for a Sunday observance that President Obama and former President George W. Bush plan to attend. Along with extra officers -- the department won't reveal an exact number -- the security also will include hundreds of surveillance cameras trained on the site, Kelly said.
The World Trade Center gathering also will showcase a new police command of 212 officers dedicated to protecting the site. The officers have received special counterterrorism training, including on how to spot people conducting reconnaissance and how to respond to suicide or truck bombers.
The command conducted drills this week on evacuating the National September 11 Memorial & Museum and its adjacent plaza, which opens Monday.
Sunday's observance is one of more than 30 commemorative events scheduled for the week that the department is laboring to protect, Kelly said.
"We don't presume that ground zero alone is a potential target," the commissioner said. "That would be shortsighted."
The department also will deploy so-called quick-strike reaction forces to respond to potential threats outside lower Manhattan, Kelly said. The teams include officers in heavy armor, bomb squad technicians and hostage negotiators who will have highway patrol escorts on standby if needed.
New Yorkers will see more officers in the subways as well. Many will concentrate on busy transportation hubs such as Grand Central Terminal, Pennsylvania Station and the Herald Square subway station.
There are other security concerns: Extra patrols also will be deployed in Queens for Sunday's U.S. Open tennis finale. And the department is gearing up for next week's U.N. General Assembly, working with the Secret Service to protect 130 heads of state and coordinate 220 motorcades, Kelly said.
"In other words, we have a lot on our plate," he said. "But we have the talent and the resources to deal with it."