High waves crashing over the car deck of a ferry that sank last week with more than 600 people on board likely caused the ship to capsize, a top Indonesian investigator said Thursday.

Similar accidents involving "roll-on, roll-off" ferries have occurred elsewhere around the world, leading experts to call for design changes in the doors and ramps that allow vehicles to drive directly on board.

More than 400 people are dead or missing and around 220 others have been rescued, including the captain of the Senopati Nusantara and 21 other survivors picked up Wednesday, said navy spokesman Lt. Col. Tony Syaiful. The captain is being questioned by authorities at an undisclosed location.

The ferry sank in the Java Sea just before midnight Friday after being pounded by waves up to 12 feet high.

"I suspect waves entered the car deck over the door and became trapped, making the vessel too heavy and unstable," Ruth Simatupang, a government investigator probing the accident, told The Associated Press.

The ferry was not overloaded, she added, citing witness testimony.

"Everyone could sit down, sleep and there was space on the car deck," she said.

Indonesia has been wracked by seasonal storms in recent weeks that have caused deadly landslides, flooding and at least six maritime accidents in different parts of the archipelago.

Survivors recalled the horror of the ferry's last minutes and the struggle to stay alive — and at least two said many of the victims were trapped after towering waves flooded the deck.

People with something to keep them afloat can survive for days in Indonesia's warm tropical waters, and officials said the search would continue until at least Sunday. But a rescue official said he thought it was unlikely many more would turn up alive.

"Looking at the weather I feel it will be difficult to find more survivors," said Mohammad Hernanto, the Surabaya search and rescue chief. "Especially since they have been thrown around in the sea without food and water."

Those rescued in recent days have been found several hundred miles from where the ferry sank.

Relatives of the missing have converged on hospitals and ports along Java's coast, hoping their loved ones will turn up alive.

The Senopati Nusantara was built in Japan in 1992 and had a capacity of 850 passengers.