Overtime for drive to remote worksite?

Q. We required three hourly employees to drive 15 hours to an out-of-state warehouse to do finishing work and only paid them for eight hours of straight time. They worked a 40 hour week, made the drive on a Saturday, worked in the warehouse Sunday through Wednesday, then made the 15-hour return drive on Thursday and reported to work on Friday. Someone at the Labor Department told me that since the drive extended past their regular workday, we were required to pay for 15 hours, and it probably should have been overtime. Is that correct?