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

Authorities are working to identify 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.