A road worker saw them alive at 11:30 pm. It would be getting dark then, though not very dark at this latitude and time of year. They were discovered dead and cold early in the morning (all discussed and backed up with news articles by others much earlier in threads). These facts suggest they were murdered sometime around as dark as it was going to get and in a timeframe where they would likely be sleeping.
Thanks for this! Clears some things up. Poor Lucas and Chynna, they were sadly easy targets for cowardly killers imo.