CHICAGO (WLS) -- Twenty-one years ago, al-Qaeda used commercial airliners to carry out coordinated terrorist attacks against the U-S.
Nearly 3,000 people were killed. In response to the September 11 attacks, the U.S. initiated the Global War on Terror.
RELATED | 9/11 attacks: By the numbers
As America marked this somber anniversary, ABC7 Chicago discussed the threats Americans face today with Tom Brady, the associate dean of the public services division and director of the Homeland Security Training Institute at the College of DuPage.
In 2001, the attacks came from outside the U.S. Brady said that's not the case in 2022.
"I believe that today, the greatest threat in the United States is domestic terrorism. What I consider to be homegrown extremism or homegrown terrorism," Brady said. "We have so many mass shootings. In 2022, we've had a total of 508 mass shootings. And mass shooting is defined as when four or more persons are shot in one incident."
WATCH: Our Chicago Part 2
Brady said domestic terrorism sometimes isn't seen by law enforcement and can fly under the radar.
"If there's someone that's planning to carry out an attack of domestic terrorism and they've had no contacts with law enforcement and they've not made their intentions known to family, friends, they are extremely dangerous," Brady said. "Law enforcement doesn't have these people on their radar, they kind of fly under the radar and that's a very dangerous thing in our country right now."
But, are we safer than we were, 21 years ago?
"There's no question that we're safer than we were on 9/11, 2001 because of the security procedures that we've put into the country," Brady said. "We've done a lot of things with airports and borders and things like that. Security's an everyday thing for us now. So we're safer from that standpoint. But can we be vulnerable, of course we can. And that's something we have to be focused on to make sure that we can be as safe as we possibly can."