Boxer obtained a letter written by a senior Southern California Edison executive from 2004 that she believes shows Edison intentionally misled the public and regulators to avoid a potentially long and costly review of four replacement steam generators before they went into service.
The nuclear power plant has not produced electricity since January 2012 after a small radiation leak led to the discovery of excessive wear inside radioactive water tubes. The plant was forced to shut down for review.
The replacement equipment Edison installed was supposed to be almost exactly the same as the older parts to ensure little to no changes to safety systems or components in the plant. But the 2004 letter reportedly shows Edison's vice president knew the replacement parts were not the same.
The senator, who also chairs the Senate Committee on Environment and Public Works, says the letter reveals "evidence of misrepresentation" and that "Edison knew they were not proceeding with a simple 'like-for-like' replacement as they later claimed."
Critics say Edison led federal authorities to believe the parts were the same in order to avoid a lengthy trial-like review that can take up to two years to complete.
Gradual wear is common in steam generator tubing, but the rate of erosion at San Onofre alarmed officials since the equipment is relatively new. Last year, federal investigators concluded that a botched computer analysis resulted in design flaws that were largely to blame for the unusual tube wear.
"SCE's own oversight of MHI's [Mitsubishi Heavy Industries] design review complied with industry standards and best practices," Pete Dietrich, SCE senior vice president, said in a statement. "SCE would never, and did not, install steam generators that it believed would impact public safety or impair reliability."
This investigation comes amid the plant's efforts to get approval from federal authorities to restart the Unit 2 reactor. Since being shut down, it has incurred more than $500 million in costs, which could threaten to shut the plant down permanently.
Edison has been pushing to restart the plant, saying it can safely operate the plant at 70-percent capacity.
The Associated Press contributed to this report.