Members of an all-male singing group from Yale University were attacked and beaten outside a New Year’s Eve party in San Francisco, and the parents of a singer who suffered a broken jaw said they are outraged no arrests have been made.

The Baker’s Dozen, an a cappella group that formed at Yale in 1947 and has performed for the president, were on a West Coast winter tour when they were invited to a party in their honor, said Reno Rapagnani, a retired San Francisco police officer whose daughter hosted the event.