Lake County Sheriff spokesman Max Duarte says one person was flown to a Denver hospital after the crash and rescue teams were searching for the fourth person aboard.
Duarte says he doesn't have information about the identities of the dead and injured.
He says the Kentucky-based chopper was training in central Colorado when it struck the 4,328-metre tall Mount Massive.
The US Forest Service says its workers building trails with a youth group heard the crash but didn't see it.
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