Three U.S. senators have called for an investigation after an Associated Press report revealed that regulators are concealing violations at deteriorating nuclear sites.

Sen. Barbara Boxer, D-Calif., sent the Government Accountability Office a letter asking the agency to investigate claims made in the article that the Nuclear Regulatory Commission has rewritten rules in order to keep plants compliant with federal regulations. Sen. Bernard Sanders, I-Vt., and Sen. Sheldon Whitehouse, D-RI, also signed the letter.

The letter builds upon a GAO report released Tuesday by Democratic U.S. Reps. Edward J. Markey of Massachusetts and Peter Welch of Vermont that criticized regulators' ability to detect and plug underground leaks that threaten the safety of nuclear plants. That report reflects a similar finding by the Associated Press, which reported that decaying pipes were contributing to nearly three quarter of U.S. nuclear sites leaking a radioactive substance called tritium.

Just as a power outage was the root cause of the core meltdowns at Fukushima, a failure of buried pipes that carry cooling water to the reactor cores could lead to a similar emergency here in the U.S., Markey said in a statement. There would be no warning because no one ever checks the integrity of these underground pipes.

The AP report also described an apparently prevalent practice known as pencil sharpening, in which regulators manipulate data to make plants appear up to code. Violations such as cracked nozzles, malfunctioning cooling components and leaky valves -- increasingly common in the nation's aging plants -- have gone unreported, the AP found.