1. A Colorado-based company helped investigate the attacks. Longmont-based engineering firm Exponent was hired by the National Institute of Standards and Technology to help investigate the collapse of the World Trade Center towers. Exponent's work included analyzing the behavior of the buildings' structural components, as well as performing computer simulations of the events leading up to the collapse.

2. A memorial to the victims was dedicated in Colorado Springs. In 2016, a 6-ton steel beam from the World Trade Center was installed at Memorial Park in Colorado Springs, surrounded by a semicircular wall bearing the names of the victims of the attacks. The monument also includes a sculpture of a bald eagle perched on the beam.

3. A Colorado Springs resident was among the passengers on Flight 93. Tom Clark, a 39-year-old software engineer, was on board Flight 93 as it was hijacked and ultimately crashed in a field in Shanksville, Pennsylvania. Clark was one of 44 people who died in the crash.

4. Denver was briefly mentioned in a key al-Qaeda document. In the so-called "Manchester Manual," an al-Qaeda training document discovered in Britain in 2000, Denver is listed as one of several potential targets for attacks on American soil. The document also references other potential targets in the U.S., as well as in Europe and Asia.

5. A unique Colorado-based memorial honors the victims. The Beamer Memorial in Boulder is dedicated to the memory of Todd Beamer, one of the passengers on Flight 93 who famously said "Let's roll" as the passengers attempted to retake control of the plane from the hijackers. The memorial, which features a large stone inscribed with Beamer's name and the words "A great American," also includes a bench with a view of the mountains that Beamer was said to have loved.