PORT WENTWORTH, Ga. -- Firefighters have pulled four bodies from the rubble of a still-burning sugar refinery that exploded Thursday night and are searching for at least four others workers who remain missing, according to Georgia's top fire official.

More than 50 other workers were injured, more than 15 critically, in the blast.

"We continue to look for survivors. We pray that there will be survivors," Georgia's Fire Commissioner John Oxendine said.

The news came after Savannah-Chatham Police Chief Michael Berkow said as many as 17 people were missing and they "expect to find fatalities inside the building."

Earlier Friday, after briefing families of the missing workers, Berkow announced that rescue efforts had shifted from a rescue to a recovery operation because the building is too unstable for firefighters to enter.

"The really hard part of this is to walk in and face a room of family members and say, 'We haven't found anyone at the scene, we know who is at the hospital, and there are people still missing,'" Berkow said.

Parts of the structure remained on fire most of the day.

Fire Chief Greg Long said he wasn't giving up hope some of the missing might still be found alive in rooms blocked by debris. The efforts were suspended at nightfall, but crews would be back on the job Saturday morning.

"I have friends that work at this plant," Long said. "I don't want to see a casualty list."

More Than 15 Critically Injured

Of nearly 100 people working in the plant when it exploded, about half were injured -- more than 30 hospitalized. Of those initially treated at Memorial University Medical, 19 were flown to Augusta to the Joseph M. Still Burn Centers at Doctors Hospital in Augusta, where 15 were reported to be in critical condition.

Seven of those who were being treated in Savannah Memorial were reported to be in critical or serious condition.

Hospital officials in Augusta told Channel 4 that most of the patients were on ventilators. Because many of the patients will need multiple transfusions, doctors put out a plea for blood to assist their efforts.

Officials had not determined what caused the explosion Thursday night but said they suspect sugar dust, which can be volatile.

"There was fire all over the building," said Nakishya Hill, a machine operator who escaped from the third floor of the refinery on the Savannah River.

"All I know is, I heard a loud boom and everything came down," said Hill, who was uninjured except for blisters on her elbow. "All I could do when I got down was take off running."

The fire was partially contained early Friday, said Capt. Matthew Stanley of the Savannah Fire Department. "We have diminished it considerably, but we're still struggling to get to parts of it," he said.

The fire had been extinguished in the area where the explosion happened, but structural damage was keeping firefighters out, Stanley said.

Firefighters hoped to enter the area Friday. Authorities also were talking with the military about bringing in Chinook helicopters to dump water on the fire, Stanley said.

'It Was Like Walking Into Hell'

Police Lt. Alan Baker and his wife, Joyce, told CNN they were among the first at the scene. Alan Baker said he went with a maintenance worker to turn off a gas main while his wife, a Red Cross first aid instructor, treated the injured.

"It was like walking into hell," Joyce Baker said. "We had approximately 13 men who were coming out and they were burned, third-degree burns on their upper bodies. And they were trying to sit down and the only thing that they wanted was to know where the friends were."

Some of the burned men had "no skin at all" and some had skin "just dripping off them," Baker said.

The explosion was as devastating as a bomb. Floors inside the plant collapsed, flames spread throughout the refinery, metal girders buckled into twisted heaps and shredded sheet metal littered the wreckage.

"I've been fire commissioner for 14 years in the state of Georgia, and when I flew over that, it was one of the worst things I had ever seen," Oxendine said.

Silo Of Sugar Explodes

Investigators had been unable to determine the cause of the explosion Friday as firefighters continued to suppress flames inside the vast refinery -- a network of warehouses, silos and buildings eight stories tall connected by corridors of sheet metal.

The plant is owned by Imperial Sugar and is known in Savannah as the Dixie Crystals plant.

"A far as we know, it was a sugar dust explosion," Imperial Sugar CEO John Sheptor said. He said it happened in a storage silo where refined sugar is stored until it is packaged.

Coast Guard Lt. Cmdr. Kevin Lynn said the river was closed to ship traffic from the Port of Savannah while the river was searched for possible victims.

"It's a large facility, and there is still a significant amount of fire," said Clayton Scott, assistant director of Chatham County Emergency Management Agency. He described the refinery as covering an area the size of a Wal-Mart Supercenter

The U.S. Chemical Safety Board said Friday it is sending an investigative team to the plant.

Sugar dust is combustible, according the U.S. Occupational Safety & Health Administration's Web site. Static electricity, sparks from metal tools or a cigarette can ignite explosions. Sugar dust is suspected of sparking a nonfatal explosion last summer at a factory in Scottsbluff, Neb., and one that killed a worker in Omaha in 1996.

Imperial Sugar, based in Sugar Land, Texas, acquired Savannah Foods & Industries, the producer of Dixie Crystals, in 1997. The acquisition doubled the size of the company, making it the largest processor and refiner of sugar in the U.S., according to the company's Web site.

Imperial markets some of the country's leading consumer brands, Imperial, Dixie Crystals and Holly, as well as supplying sugar and sweetener products to industrial food manufacturers.