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According to an announcement made to the crowd, Nelson, was "suffering from breathing problems due to the high altitude and emphysema" and taken to a near by hospital.

He was scheduled for a fundraiser for the Denver Dumb Friends League called Lulu's Barkin BBQ.

A source tells 9NEWS Nelson's band and back-up singers arrived at the venue, but Nelson never made it.

A spokesperson for AEG Live, the promoters that usually handle Nelson's performances, offered no comment on Sunday.