Holder spoke Monday at the International Association of Chiefs of Police Conference being held in downtown Denver.

"Few casual observers would cite a mid-sized city like Aurora, Colorado, as a central battleground in the fight against terrorists," said an early copy of Holder's speech. "We know that every city and town in America - and therefore every law enforcement official in America - has a role to play in the fight against terrorism."

Holder said the plan involved detonating explosives in the United States. Sources tell 9Wants to Know Arapahoe County resident Najibullah Zazi was one of the masterminds behind the plan.

The FBI has visited several businesses in the metro area including home improvement and beauty supply stores, along with U-Haul dealers asking employees to identify names and photographs provided by agents.

The FBI has obtained videotape of Zazi buying beauty supplies containing acetone and hydrogen peroxide at The Beauty Supply Warehouse in Aurora.

Sources also tell 9NEWS the FBI has videotape of Zazi and two or three others in the U-Haul dealer at 750 S. Buckley Rd. in Aurora trying to rent trucks.

FBI agents arrested Zazi and his father, Mohammed Zazi, at their home near Smoky Hill Road and E-470 Sept. 19.

Zazi faces one charge of conspiracy to use weapons of mass destruction. A grand jury charged him Sept. 24 in the Eastern District of New York.

Zazi has pleaded not guilty to the charge. If convicted, he could spend the rest of his life in prison. He's currently being held at the Metropolitan Detention Center in Brooklyn, N.Y.

His next court appearance is set for Dec. 3.

If you have a tip on this story for investigative reporter Jace Larson, e-mail him at jace.larson@9news.com or call 303-871-1432.