Minneapolis, Minnesota was also mentioned in a presentation Congress made to the House Homeland Security Committee last week.
9NEWS spoke to one of Denver's top terrorism experts. He says the US government needs to shift its focus from high-profile targets like the White House to Main Street.
Metro State Professor Norman Provizer defines homegrown terrorism as: "A terrorist threat that grows up out of your domestic population as opposed to being imported.'
No longer do just the big city targets like Washington, D.C. and New York City need to prepare. Now locations - that are seemingly off the terrorism grid - are being told to prepare.
"Now it's a question of be aware, no matter where you are, as opposed to be aware if you're going to New York City," Provizer said.
Since 9/11, there have been 43 homegrown terrorists linked with Al-Qaeda and nearly half of those arrests have been made in the last year.
In the past, terror fighting efforts relied strictly on military and national intelligence agencies. Provizer says it's now on the shoulders of local law enforcement, public safety personnel, and even hometown residents.
So why did Denver make the list?
"There are cities that have had incidents, such as Denver, and those become interesting places to use as your illustrations of the point," Provizer said.
There have been incidents of homegrown radicals being arrested for ties to terrorist groups. For instance, you may remember Najibullah Zazi, the airport shuttle driver from the Denver area accused of having ties to Al-qaeda.
Faisal Shahzad was another homegrown terrorist. Shahzad was the botched Times Square Bombing suspect whose wife had a degree from the University of Colorado.
Proviser says there's been a rise in homegrown threats because even though they're less sophisticated, they're harder to foil.
"The intensity of the attacks are probably going to be much lower scale, but the possibility of carrying them out is higher scale," he says.
The professor says people need to strike a delicate balance: staying alert without acting panicked.
"The purpose of terrorism is to create terror, and if in fact, you act terrorized all the time, then in fact, you have fulfilled one of the goals of terrorism," Provizer said.
Many of the nation's best-known terrorists are being held at the Federal Supermax Prison in the small Southeastern Colorado town of Florence.
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