Crews were working Thursday to clean up 15,000 gallons of oil that spilled onto a Minnesota field after a mile-long train derailed.
It was not immediately clear whether the Canadian Pacific train was transporting oil from the Alberta tar sands, but the spill will add fuel to the debate over the controversial Keystone XL pipeline.
Frigid temperatures helped contain environmental damage after 14 train cars fell off the tracks and three began leaking oil on Wednesday.
"The site is still frozen and covered with quite a lot of snow, which helped prevent any oil from moving down the ditch or soaking into the soil," said Dan Olson of the Minnesota Pollution Control Agency.
"The accident currently poses no threat to either surface or ground waters," he said, but warned the chill had also made it harder to collect the oil, which has thickened into a tar-like consistency mixed with snow.
Crews are digging up the oily snow and piling it into a lined ditch until it can be hauled away, which will likely not happen for another day or two.
It will take a few weeks to drain the oil from 11 of the derailed tanker cars because they need to be heated with steam before crude can be pumped out.
The cause of the accident has not yet been determined but the line was reopened on Thursday after track repairs and a full inspection, Canadian Pacific said.
The mixed-freight train, with a total of 94 cars, originated in Alberta and was bound for the Chicago area.
A spokesman for Canadian Pacific told AFP he did not know if the oil came from the province's tar sands or from conventional drilling sites.
The spill had initially been estimated at 20,000 to 30,000 gallons.
No injuries were reported as a result of accident, which was in a rural area about a mile north of Parkers Prairie in west central Minnesota, the Otter Tail county sheriff's office said.
Environmental activists are preparing to flood an April 18 public hearing in Nebraska to discuss the controversial $5.3 billion Canada-to-Texas Keystone XL pipeline.
The US State Department released a draft environmental impact statement on March 1 suggesting the rerouted pipeline, which would transport some 830,000 barrels a day, would have no major impact on the environment.
Critics contend the heavy tar sand oil would be nearly impossible to clean up if it were to spill into one of the more than 1,000 waterways that will be traversed by the pipeline, because it sinks instead of floats.