Police respond to a shooting near 1531 Wynkoop in downtown Denver Tuesday afternoon, June 28, 2016.
Several others were inside the office building and sheltered in place.
Denver police tweeted out an update on the incident near the EPA offices and Office Depot.
The building houses several left-leaning political and environmental organizations, according to The Denver Post.
SWAT officers were seen on nearby rooftops.
Investigators with Homeland Security were also on scene.
The shooter first tried to enter the building through a doorway in an alley, but it was locked to non-employees, Saunier said. She was critically injured and underwent surgery. She died Wednesday morning at a Denver hospital. Police said there were about 120 witnesses, and some tried to intervene. "He was becoming more and more mentally unstable".
He was banned from the airport in Buena Vista based on some comments he had made, Saunier said.
"Definitely saw quite a few police officers exit their vehicles with their guns drawn", said John Guenther, who was at a bookstore at the time.