From the Exxon press statement:
Based on the metallurgical analysis, the independent laboratory concluded that the root cause of the failure can be attributed to original manufacturing defects — namely hook cracks near the seam.
Additional contributing factors include atypical pipe properties, such as extremely low impact toughness and elongation properties across the ERW [electric resistance welded] seam.
There are no findings that indicate internal or external corrosion contributed to the failure.
Their claim about there being no corrosion is important because there has been concern that the abrasive tar sands oil being transported in this pipeline may have contributed to the spill. The pipe was not originally designed to carry that type of fuel.
John Upton at Grist notes that this raises more concerns about the entire Pegasus pipeline, which was installed in the 1940s.
The findings bring into question the integrity of the entire Pegasus pipeline system — and other oil pipelines that crisscross the nation. The Pegasus system, which runs from Illinois to Texas, was laid in 1947 and 1948. The pipeline manufacturer, Ohio-based Youngstown Sheet and Tube Co., is no longer in business but was reportedly one of the leading suppliers of pipelines in the 1940s.