Routt County Coroner Robert Ryg said that 37-year-old Austin Eubanks died overnight at his Steamboat Springs home.
13 people were killed when two students at the High School in Colorado in 1999.
The FBI, who had been hunting the 'armed and dangerous' teenager, tweeted 'there is no longer a threat to the community'.
Its impact is perhaps magnified by the lasting memories of the atrocity, because almost all of the teenagers' attack was caught on camera.
Police say the 18-year-old is armed, extremely dangerous and should not be approached.