ALICEVILLE, Ala. (AP) — Environmental regulators promised an aggressive cleanup after a tanker train hauling 2.9 million gallons of crude oil derailed and burned in a west Alabama swamp in early November amid a string of North American oil train crashes.

So why is dark, smelly crude oil still oozing into the water four months later?

The isolated wetland smelled like a garage when a reporter from The Associated Press visited last week, and the charred skeletons of burned trees rose out of water covered with an iridescent sheen and swirling, weathered oil. A snake and a few minnows were some of the few signs of life.

An environmental group now says it has found ominous traces of oil moving downstream along an unnamed tributary toward a big creek and the Tombigbee River, less than 3 miles away. And the mayor of a North Dakota town where a similar crash occurred in December fears ongoing oil pollution problems in his community, too.

As the nation considers new means of transporting fuel over long distances, critics of crude oil trains have cited the Alabama derailment as an example of what can go wrong when tanker cars carrying millions of gallons of so-called Bakken crude leave the tracks. Questions about the effectiveness of the Alabama cleanup come as the National Transportation Safety Board considers tighter rules for the rail transportation of Bakken oil, which is produced mainly by the fracking process in the Bakken region of North Dakota and Montana. Oil production is increasing there, boosting the amount of oil being transported across the country.

Environmentalist John Wathen, who has conducted tests and monitored the Alabama site for months for Waterkeeper Alliance, said Genesee & Wyoming railroad and regulators did the bare minimum to spruce up an isolated, rural site and left once the tracks were repaired so trains could run again.

"I believe they really thought that because it's out of sight, out of mind, out in the middle of a swamp, that nobody was going to pay attention," said Wathen.

Regulators and the company deny any such thing occurred, however.

The Environmental Protection Agency and the Alabama Department of Environmental Management, which oversaw the cleanup, say more than 10,700 gallons of oil were skimmed from the water after the derailment, and workers collected about 203,000 gallons of oil from damaged rail cars using pumps. Another 290 cubic yards of oily dirt was excavated with heavy equipment, or enough to cover a basketball court with soil nearly 2 feet deep.

Yet four months later, officials still say no one knows exactly how much oil was spilled. That's mainly because an unknown amount of oil burned in a series of explosions and a huge fire that lasted for hours after the crash. Since no one knows how much oil burned, officials also can't say how much oil may be in the swamp.

About a month after the crash, the head of Alabama's environmental agency, Lance LeFleur, promised "aggressive recovery operations" in a written assessment for a state oversight commission. He said the oil had been contained in a "timely" manner and none had left the wetlands.

Michael Williams, a spokesman for the Connecticut-based Genesee & Wyoming, which owns the short-line Alabama & Gulf Coast Railway line where the crash occurred, said the company is still monitoring the site closely and maintaining a system of barriers meant to keep oil from spreading. The work is continuous, he said.

But regulators and the railroad confirm one of Wathen's worst fears: That environmental agencies let the railroad repair the badly damaged rail bed and lay new tracks before all the spilled oil was removed. Wathen calls the move a mistake that's behind the continuing seepage of oil into the water.

"I do agree that they needed to get the rail cars out. But there were other ways to do it," said Wathen. "Those would have been more expensive."

James Pinkney, an EPA spokesman in Atlanta, said the rail line had to be fixed quickly to remove oil and damaged rail cars that still contained crude from the wetland.

Agencies are now working with the company and its contractors to recover the remaining oil trapped in the rail bed, but it's unclear when or how that might happen.

"The EPA and ADEM are continuing to work together to insure all recoverable oil is removed from the site," Pinkney in a written response to questions.

Ed Overton, an environmental sciences professor at Louisiana State University, said spilled crude can linger at a site indefinitely if it's buried in the ground. Depending on the amount of oil that remains, he said, containment devices may be needed in the swamp for at least a couple of years.

But Bakken crude evaporates quickly once exposed to air because of its composition, said Overton, so the fact that oil remains in the swamp isn't "the end of the world."

"It's going to look bad for awhile," he said. "It's amazing how quickly Mother Nature can handle such things, but it will take time."

The cause of the derailment — which happened at a wooden trestle that was destroyed by the flames and since has been replaced by buried culverts that let water flow underneath the tracks — remains under investigation by the Federal Railroad Administration.

The crash site appears in better shape now than right after the derailment, partly because burned tanker cars misshapen by explosions are gone. Much of the water surrounding the site appears clear, and the odor from the site isn't bad enough to reach the home of Leila Hudgins, just a few hundred yards away.

"I haven't smelled anything," said Hudgins. "They did a good job. They hauled off truckload after truckload."

The crash site, located off an old dirt road and a new one that was built during the response, is accessible both by car and foot, but Hudgins said she hasn't looked closely at the spot where it happened.

The railroad said testing hasn't detected any groundwater contamination, and EPA said air monitoring ended about a month after the crash when it became apparent there were no airborne health hazards.

Still, questions linger. Wathen said he has been taking water samples several hundred yards downstream from the crash site and has detected the chemical fingerprint of so-called Bakken crude, which the train was carrying with it derailed.

"There's no question it is outside their containment area, and I think it's even further away," said Wathen. "This is an environmental disaster that could go on for years."

The Alabama train was on a southbound run when it derailed less than 3 miles south of Aliceville, a town of about 2,400 people near the Mississippi line. Another oil train derailed and burned in December at Casselton, N.D., and 47 people died in July when a train carrying Bakken oil exploded and burned in Quebec.

The mayor of Casselton, Ed McConnell said he has been keeping up with the Alabama cleanup because spilled oil also was buried under the rebuilt railroad tracks near his town of 2,400 people. He worries that oil will reappear on the ground at Casselton as the spring thaw begins in coming weeks.

"It's still in the ground here, too," said McConnell. "They've hauled a lot of dirt and stuff out. But they covered up the (oily) dirt before getting it all up and rebuilt the track to get it going."

Alabama's environmental agency said it still regularly visits the wreck site, which is encircled with the same sort of absorbent fencing, oil-snaring pom-poms and plastic barriers that were used on the Gulf Coast after the BP well blowout in 2010.

Once the "emergency" phase ends, the state environmental agency will install wells to monitor groundwater, said spokesman Jerome Hand.

Government regulators will approve any plans for removing remaining oil from the site, he said.