Enbridge Energy said an explosion in November that killed two workers in northwestern Minnesota happened after oil mist escaped from a coupling and was ignited by a heater.
Enbridge did its own investigation after the Nov. 28 blast. A federal investigation is not yet finished.
Enbridge said an exposed pipeline was not anchored properly. When it was filled with oil, the couplings that held the pipe segments allowed an oil mist to escape. The company said the mist was probably ignited by an open-flame heater used to warm workers.
An Enbridge spokeswoman says the explosion is not the fault of any employee or crew member.
She said Enbridge employees were faced with an uncommon combination of factors at the site, including a bend in the pipeline and excavations around it. The company will change its written procedures and training to prevent similar accidents in the future.