1988, would there have been low altitude training in the Rockies after dark, sonic boom's?

If so, would there be a bright flash?

West slope Colorado, October. We were in a creek canyon at approx. 9,000' & surrounded by peaks to 12,000, it was well after dark. One hell of a boom & flash. Sonic boom was all any of us could think of but never heard a jet.

Any thoughts?