Investigators did not know how much had spilled, but an oily sheen was reported as far as three miles downriver of Vicksburg after the 1:12 a.m. incident, said Lt. Ryan Gomez of the Coast Guard's office in Memphis, Tenn. Authorities were still trying to determine the source of the leak, but it appeared to be coming from one or two tanks located at the stern of the first barge, Gomez said. 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. "Investigators are still trying to figure out what happened," he said. United States Environmental Services, a response-and-remediation company, was working to contain the oil with booms before collecting it and transferring it to one of the barge's undamaged tanks, then ultimately to a separate barge, 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. "It's still considered an active leak," Gomez said. We don't have an estimate or accurate amount of what was released. Railroad traffic was allowed to continue after the bridge was found safe for trains, Petty Officer Carlos Vega said. 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 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. McConnaughey reported from New Orleans. Associated Press Writer Lisa J. Adams in Atlanta contributed to this report.