Plains Midstream Canada said a spill on its Rainbow pipeline in Alberta might have been caused by loose soil near the line.
"It was obvious to us onsite that there was improper compaction," the Platts news service quoted Stephen Bart, vice president of operations for Plains, as saying from Calgary.
Canadian regulators estimate that around 28,000 barrels of oil spilled from the pipeline in late April, making it the largest spill in the region since 1975.
Plains was scrutinized in 2006 for a 7,500-barrel leak at a section of the same pipeline in Edmonton. Investigators blamed stress and corrosion for the 2006 leak. Bart said his company didn't find evidence of cracking or corrosion during mid-April inspections, however.
Platts notes oil from the region is being rerouted. Most of the spill last week was contained in the area, though some of it had reached a wetland near the site. None of the oil had reached flowing water, however.