A barge laden with 80,000 gallons of oil struck a railroad bridge in Vicksburg, Miss., over the weekend, spilling light crude into the Mississippi River and closing the waterway for miles each way, the Coast Guard said. A second barge was damaged.

Although an oily sheen was reported up to three miles downriver from Vicksburg, investigators were uncertain how much of the oil had spilled when the bridge was hit early Sunday, Coast Guard spokesman Lt. Ryan Gomez said.

"Investigators are still trying to figure out what happened," he said by telephone from Coast Guard offices in Memphis, Tenn.

The oil sheen from Sunday's incident was unlikely to pose a threat to the Gulf of Mexico, located more than 340 river miles south of Vicksburg.

Authorities were still trying to pinpoint the leak's source, but it appeared to be coming from one or two tanks located at the stern of the first barge, Gomez added. He said there was no indication that any oil was leaking from the second vessel, and said it was still unclear whether the second barge also hit the bridge or was damaged through a collision with the first.

United States Environmental Services, a response-and-remediation company, was working to contain the oil with booms before collecting it, Gomez said.

He could not say how long the river would remain closed in the area. Five northbound and two southbound vessels were waiting to pass, he said. A message seeking a Coast Guard update early Monday was not immediately returned.

Railroad traffic was allowed to continue after the bridge was found safe for trains, Petty Officer Carlos Vega said Sunday.

The barges are owned by Third Coast Towing LLC, Gomez said. According to a website listed under that name, the company is located in Corpus Christi, Texas. No one answered the telephone at the company Sunday night.

Both vessels were being pushed by the tugboat Nature's Way Endeavor. The website for Nature's Way Marine LLC of Theodore, Ala., identifies the vessel as a 3,000-horsepower, 90-foot-long boat. It was built in 1974 and underwent a complete rebuild in 2011, according to the company.

A company manager referred calls to the Coast Guard command center at Vicksburg.

The last time an oil spill closed a portion of the lower Mississippi River, it was for less than a day last February after an oil barge and a construction barge collided, spilling less than 10,000 gallons of oil. In 2008, a fuel barge collided with a tanker and broke in half, dumping 283,000 gallons of heavy crude into the waterway, and closing the river for six days.

Residents and businesses in Gulf Coast states are still recovering from the April 2010 explosion of the Deepwater Horizon offshore drilling rig, which killed 11 workers and spewed more than 200 million gallons of oil into the Gulf.

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McConnaughey reported from New Orleans. Associated Press Writer Lisa J. Adams in Atlanta contributed to this report.