Breazeale said the damaged barge had eight tanks and each tank could hold 80,000 gallons of oil. He said the investigation had shown that one tank was pierced above the water line.
"It's not leaking that much oil," Breazeale said, though the exact amount was not known.
He also said it was uncertain how long the river would remain closed.
Coast Guard spokesman Jonathan Lally said crews are working on skimming oil that is contained in the boom and that he, too, was unsure much oil had leaked.
Tugs were holding the barge near shore on the Louisiana side of the river, south of the bridge it hit and directly across from Vicksburg's Riverwalk Casino.
Orange containment boom was stretched across part of the river downstream from the barge, and a small boat appeared to patrol the area.
About 10 to 12 northbound vessels and 10 to 12 southbound vessels were waiting to pass Monday, according to Breazeale.
Coast Guard spokesman Lt. Ryan Gomez said investigators reported that a towboat or tug was pushing two tank barges when the collision occurred about 1:30 a.m. Sunday.
The second barge was damaged, Gomez said, and authorities inspected and cleared the railroad bridge afterward.
The oily sheen was reported up to three miles downriver from the bridge at Vicksburg on Sunday. Gomez said crews have laid down a boom and also a secondary boom. They also were using a rotating skimmer device to sweep up oily water in the river.
"They have the boom to contain any crude oil that's leaking out of the barge. They have a secondary boom to corral any crude oil that gets past the first boom," he said.
He said crews also were in the process of working to transfer the remaining oil.
"They are continuing to try to remove the product from the damaged tank to one of the non-damaged tanks on the same barge," he added. "The ultimate goal is to transfer all of the crude to a different barge."
He said the barge was southbound at the time of the collision, but investigators were still trying to figure out exactly what happened Sunday.
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.
But it appeared to be coming from one or two tanks located at the stern of the first barge, Gomez had said previously. He said that there was no indication that any oil was leaking from the second barge and that 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.
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. A woman who answered the phone Monday at Third Coast Towing LLC declined to comment.
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.
Barges hit the Vicksburg bridges, the Interstate 20 bridge and the old U.S. 80. bridge, from time to time, and a sharp bend in the river there is often blamed. In March 2012, barges carrying grain broke loose and hit the old U.S. 80 bridge. A barge scraped the bridge in July 2011.
On March 23, 2011, several barges broke loose and some hit the U.S. 80 bridge and Interstate 20 bridge. One was hung up on the I-20 bridge for about three weeks before it was removed.
Associated Press writers Janet McConnaughey in New Orleans and Lisa J. Adams in Atlanta contributed to this report.