Georgia authorities said 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.

"It is a structural failure. There should be very, very little maintenance to an aluminium gangway like that, but we'll see what the investigation unfolds," Georgia Department of Natural Resources Commissioner Walter Rabon said at a news conference.

The gangway was installed in 2021, authorities said.

Rabon said three people remained hospitalised in critical condition from the weekend's collapse.

Rabon said "upwards of 40 people" were on the gangway when the "catastrophic failure" occurred, and at least 20 people fell into the water. The gangway connected an outer dock where people board the ferry to another dock onshore.

None of the seven people killed were residents of the island, Rabon said. Eight people were taken to hospitals, at least six of them were initially reported to have critical injuries.

Crews from the US Coast Guard, the McIntosh County Fire Department, the Georgia Department of Natural Resources and others searched the water, according to Natural Resources spokesperson Tyler Jones. The agency operates the dock and ferry boats that transport people between the island and the mainland.

A team of engineers and construction specialists were on site today to begin investigating why the walkway failed, Jones said.

"There was no collision" with a boat or anything else, Jones said. "The thing just collapsed. We don't know why."

Helicopters and boats with side-scanning sonar were used in the search, according to a Department of Natural Resources statement.

US President Joe Biden said federal officials were ready to provide any assistance needed.

Ed Grovner was working as senior mate on one of the ferries taking people between the island and the mainland.

He told The Associated Press the ferry pulled up to the dock a short time after the collapse and crew members saw orange lifejackets bobbing in the water that had been tossed in to help people who had fallen. Grovner said he and other crew members tried to help a man and a woman, with someone administering CPR, but they were already dead.

"I couldn't sleep last night," Grovner said. "My wife said I was sleeping, I was hollering in my sleep, saying, 'I'm going to save you. I'm going to save you. I'm going to get you'."

He sighed deeply and said: "I wish I could've did more."

Resident Reginald Hall was among those who charged into the water, where an outgoing tide created a strong current that was pulling victims toward the ocean.

Hall said he was handed a two-year-old child and passed her along a chain of bystanders to shore, roughly 55 metres away. He then helped carry blanket-wrapped bodies.

"It was chaotic," Hall said. "It was horrible."