ExxonMobil says the fault of the pipeline rupture that dumped an estimated 150,000 gallons of crude oil into a Mayflower neighborhood March 29 lies with the pipe's manufacturer, a company that is no longer in business.The segment of ruptured pipeline that contained a 22-foot gash was sent to Hurst Metallurgical Laboratory in Texas. The lab's report identifies the cause of the rupture as "original manufacturing defects," according to ExxonMobil spokesman Aaron Stryk. Stryk said Wednesday the defects include "hook cracks" near the pipe's seam. He said the report identifies the manufacturing defects as the "root cause" of the failure. Hook cracks, Stryk explained, are metal separations at the edge of a metal plate that occur during welding. Additional contributing factors named in a statement provided by Exxon Wednesday include "atypical pipe properties, such as extremely low impact toughness and elongation properties across the [electric resistance welding] seam." Stryk explained that a trash bag would have high impact toughness and elongation properties. The metal pipe had low impact toughness and low elongation properties, which he said Exxon is attributing to original manufacturing defects. The original manufacturer was Youngstown Sheet and Tube Co. in Youngstown, Ohio, Stryk said. He said the company has been out of business many years, but it was one of the largest pipe manufacturers in the 1940s when Exxon's Pegasus Pipeline was built.Metallurgical testing to determine the cause of the rupture and crude oil release was part of the company's corrective action order, a series of steps enforced by the federal Pipeline and Hazardous Materials Safety Administration.The order requires ExxonMobile Pipeline Company to take corrective action to protect the public, property, and the environment from hazards associated with the oil spill and pipeline failure. The metallurgical report, the third in the series of corrective actions, was due Wednesday at the close of a third extension granted by PHMSA.The corrective action order states Exxon was to complete the testing and analysis 45 days within receipt of the order issued April 2. Stryk said Exxon received a preliminary draft of the report in late May and asked for a four week extension to conduct supplemental testing. "The supplemental testing will be used in developing a remedial work plan," he said. The remedial work plan is step four in the corrective action order, and states Exxon must provide verification of the integrity of the pipeline while addressing all factors known or suspected in the failure. Exxon must verify any repair information and the results of in-line inspections. The last in-line inspection was conducted in February. ExxonMobil Pipeline Company executive Karen Tyrone said in a recent interview the results of February's in-line inspection are not available yet. "I know it's hard to understand and I'm sympathetic to that, but these tools generate a significant amount of data that has to be run through programs and computer analysis with tools that tell us where to go to evaluate the tool run," Tyrone said.She said the company is verifying the tool run by doing "validation digs" at the pipeline.That element of the corrective action order is due within 90 days of providing the metallurgical report that was given to PHMSA Wednesday. The federal pipeline safety administration did not release the report to the public Wednesday citing an ongoing investigation. Stryk said it was up to PHMSA to release the report and did not release it, but said there are no findings that indicate internal or external corrosion contributed to the pipeline's failure. In PHMSA's preliminary findings, it is noted that the company reversed the system flow in the pipeline in 2006, which "can affect the hydraulic stress demands on the pipeline."Findings state the pipeline contains both seamless pipe and low frequency electric resistance welded pipe. The entire pipeline, which runs from Illinois to Texas, is still out of service. Tyrone said the rupture site has been repaired with a new piece of pipe, and that there is no retirement age for the Pegasus Pipeline. The pipeline was constructed in 1947 and 1948, according to PHMSA. Stryk said Wednesday once the remedial work plan is completed within the 90-day period, it does not mean the pipeline will be restarted. "There are a lot of steps to take until that begins," he said. Steps listed later in the corrective action order require Exxon to restrict pressure in the pipeline to 80 percent of the actual operating pressure until PHMSA's director removes the restriction. Exxon must also submit quarterly reports with testing and evaluation data for an undesignated period of time.