Authorities are investigating the cause of a partial ferry dock collapse on Georgia’s Sapelo Island that turned a day of celebration into tragedy, leaving at least seven dead and six critically injured as crowds gathered for a cultural festival.

Georgia authorities said Sunday they are investigating the “catastrophic failure” of a dock gangway that collapsed and killed seven on Sapelo Island, where crowds had gathered for a fall celebration by the island’s tiny Gullah-Geechee community of Black slave descendants.