Four crew members are still unaccounted for after a cargo ship overturned in the St. Simons Sound Sunday morning, according to the Coast Guard.

It was initially deemed unsafe for Coast Guard crews to continue searching on board due to black smoke and flames seen coming out of the cargo hold.

UPDATE: Final crew member rescued from overturned cargo ship in St. Simons Sound

Black smoke is no longer seen coming from the vessel, but the ship is continuing to shift, making it unstable, the Coast Guard said. The Coast Guard hopes to resume rescue efforts shortly. 

The Golden Ray was conducting a starboard turn to depart the harbor around 2 a.m. when it started listing over on the port side.

The 656-foot, 71,000-ton vehicle carrier vessel has a crew size of 24 people; 23 crew members and a pilot. Twenty of the 24 crew members have been safely rescued and looked over by medical personnel.

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The Coast Guard is working to stabilize the vessel so that the search for the four missing crew members can continue. Once the vessel has been stabilized, they will determine the best way to continue rescue operations.

All vessel traffic through the Port of Brunswick had been suspended unless approved through the USCG Captain of the Port.

At this time, there have been no reports of injuries and it is not yet known what led to the ship listing. The cause of the incident is still being investigated. 

The National Transportation Safety Board is assisting the Coast Guard with the investigation into this incident. 

The Georgia Department of Natural Resources is monitoring coastal environmental conditions. An emergency spill response crew is stationed nearby at the city of Brunswick boat ramp beneath the Sidney Lanier Bridge, according to the Coast Guard. They are preparing to deploy spill containment booms around the Golden Ray. Photos from the Georgia Department of Natural Resources show fluid leaking from the ship.

RELATED: Swimming Advisory issued for St. Simons, Jekyll Island beaches due to overturned cargo ship in St. Simons Sound

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Rod Sullivan, a maritime law expert in Jacksonville, says the ship was under the control of a harbor pilot – the local mariners who bring big cargo vessels in and out of ports.

Because of that, he says “the odds that this was a navigational error is virtually zero. Probably what it was was a mechanical failure, like a steering failure.”

For instance, Sullivan says, if the rudder failed and turned hard, the ship could have run aground, causing it to list and the cargo to shift.