The VA document says Riehl was hospitalized at the veterans medical center in Sheridan, Wyoming, in April 2014 after a psychotic episode.
"You know, I'm not here to hurt anybody but yes I do have firearms,” Matthew Riehl can be heard saying before opening fire on police.
University of Wyoming officers called police in Lone Tree, Colorado, in mid-November to warn them about Riehl, suggesting his rants were indicative of mental illness.
Telluride High School was closed Monday as various law enforcement agencies searched the building.
The shooting appears random and there are no indications that it was an act of terror, said Thornton police spokesman Victor Avila.
Experts say just calling 911 may not be enough to survive a gun attack in the workplace.
We talk about what you told us you want and don't want from the media after a mass shooting in Colorado -- it's a question we posed through our Public Insight Network. Among the people who responded was Coni Sanders, whose father was killed at Columbine High School in 1999. She thinks telling these stories is important, "but it needs to be done so responsibility, meaning there be a focus on prevention, there be brief, little to no mention of the killers' names, especially showing their faces."
Also on Thursday, a rare coalition of American Indian nations, including the Ute Mountain Utes of Colorado, seek a new national monument across the border in Utah.
Backers say school districts need to be held accountable and not shielded by governmental immunity.
The Senate Judiciary Committee passed the bill after passionate testimony from supporters including the parents of Claire Davis, who was shot to death at Arapahoe High School in 2013.