Alleged harassment after off-site party: Must we investigate?

Q. We have a company party each year at an off-site location. We reimburse employees who live 50 miles away half of their hotel room for the night. What is our responsibility or liability if alleged harassment takes place after the party is over and occurs in one of the rooms we partially paid for? The people involved evidently had a lot of alcohol to drink and I think the event happened early in the morning, possibly around 2 or 3 am. Do we investigate the harassment claim or not since it happened away from work and on a weekend? – Robin, Iowa