A ferry dock collapse on Sapelo Island shook the Gullah Geechee community on Saturday. Here's more on the Gulla Geechee people and their history.

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.

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.