The Douglas County Sheriff's Office say the pileup happened on northbound Interstate 25 near Larkspur around 8:45 a.m.
"This was really hard snow. This is something where people were going way too fast for the conditions. They just weren't paying attention," driver Brad Simpson said.
The Colorado State Patrol says 34 vehicles, including four semi-trucks, were involved in three separate crashes. People from two cars required extrications.
"The cars that were in front of us were pretty much spinning out of control, doing 360s, and we just managed to pull over to the side and avoided it all," Carl Howell said.
Troopers say one of the semi-trucks was leaking diesel fuel, which required a HazMat crew to respond to the scene to siphon fuel from the semi.
Authorities say at least 12 people were treated at Sky Ridge Medical Center and seven have been released; two people were transported to Penrose Hospital, two were taken to Swedish Medical Center and one person was transported to Memorial Hospital North.
Troopers say all injuries were non life-threatening.
"I had started to swerve and fortunately I came up a little bit short," Jim Butler said. "I'm one of the lucky ones. I'm very blessed, very fortunate."
Authorities say school buses were used to transport non-injured people from the scene of the crash to a shelter at the New Covenant Church in Larkspur.
All lanes of I-25 are back open.
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