Story highlights The Mobile River barge fires are out, the U.S. coast Guard says

A spark ignited natural gas vapors, a Coast Guard official tells CNN affiliate WALA

Three injured workers are in critical condition with severe burns, a hospital says

The explosion happened near where the stricken Carnival Triumph is being repaired

A fire that sparked huge explosions and injured three workers on a pair of Mobile River barges is out, the U.S. Coast Guard said Thursday.

Three workers who were preparing the barges for reloading Wednesday night were severely burned, according to Mobile fire officials.

They were in critical condition Thursday at the University of South Alabama Medical Center, according to a spokesman. Authorities don't believe any workers are missing.

The barge was empty at the time of the initial explosion Wednesday night, but authorities believe a spark ignited vapors in it, causing an explosion that ignited a second barge, CNN affiliate WALA TV reported, citing Coast Guard Lt. Mike Clausen.

At least seven explosions rocked the barges, according to the Coast Guard.

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At the Fort Conde Inn across the river they sounded like "planes above you dropping bombs," general manager Alan Waugh told WALA Wednesday.

"We were up on a second floor balcony and the sky lit up in orange and yellow," he said.

With the fire out, the Coast Guard has lifted the safety zone around the scene, allowing the shipping channel to reopen, the agency said in a news release.

The barges are moored across the river from the Carnival Triumph , the cruise ship that lost power in February and drifted in the Gulf of Mexico for days before being towed to Mobile for repairs.

At least 500 employees working on the ship were evacuated, Huffman said.