Updated

As the United States on Sunday prepares to mark the 15th anniversary of the 9-11 terror attacks, Homeland Security Secretary Jeh Johnson says terror plots, unbeknownst to the public, have been disrupted “all of the time” since he started running the agency.

“We have prevented a number of actors from carrying out their intent at the very latest stages of their plotting and planning,” Johnson told Fox News on Friday.

Nearly 3,000 people were killed on Sept. 11, 2001, when al Qaeda high-jackers took down four U.S. commercial jets, including two flown into the twin World Trade Center towers.

Johnson also said the new wave of potential terror strikes is coming with little warning and in more undetectable ways.

“9-11 was the prime example of a terrorist-directed attack,” he said. “We now live with the prospect of terrorist-inspired attacks in which ISIL, al Qaeda try to literally reach into our homeland through the internet, through social media.”

Johnson made his remarks as federal employees for the first time in 15 years are working in offices at the Trade Center, rebuilt in Lower Manhattan with a memorial to the victims of the attack, which toppled the two towers.

“It goes to our resilience as Americans, as New Yorkers, as people of the free world -- that we won't let somebody else dictate what we do," said Roger Parrino, a senior counsel for Johnson.

Johnson told Fox News that U.S. intelligence officials have no information about a specific or credible domestic threat Sunday. However, he suggested that such advanced warnings are no longer what they were to the intelligence community.

“In this environment, it's important to, frankly, not get hung up on that -- when you're dealing with terrorist-inspired attacks that could strike with little or no notice,” he said. “The cop on the beat could be the one to detect the next terrorist attack on the U.S.”

Johnson argued the new faces of domestic terror are people in the United States like husband-wife team Omar Farook and Tashfeen Malik, who last year fatally shot 14 people in San Bernardino, Calif., and Omar Mateen, who in June shot and killed 49 people inside an Orlando, Fla., nightclub.

They struck with essentially no advanced notice and without direct support from members of a terror network.

All three also appeared self-radicalized, meaning they became radical Islamic terrorists via internet sites, compared to going to the Middle East or perhaps talking with radial clerics in the U.S.

“Nothing can prepare you for that moment,” Johnson said.

Still, Johnson acknowledges “there were definitely signs” that at least Mateen was preparing for an attack. And he urged members of the country's Muslim community and other Americans to notify authorities.

“If people close to someone who carries out an act of terror can be encouraged to come forward and say something, that does make a difference,” he said.

Johnson declined to say the whether the Obama administration bringing refugees from Syria into the United States increases the domestic terror threat, saying he doesn’t comment on what the 2016 White House candidates say.

However, he suggested he opposes Republican nominee Donald Trump’s plan in the wake of several terror attacks linked to Islamic terrorism to temporarily ban Muslims from entering the U.S.

“To vilify and isolate American Muslim communities is very much counter to our homeland security and national security efforts,” said Johnson, who described the surreal, life-changing experience of seeing the 9-11 attacks while practicing law in New York.

“It was a moment in my life when my brain did not believe what my eyes were seeing,” he said. “I kept thinking out of the rubble and the smoke would emerge the towers. But it didn't happen, and it changed our nation and our world.”