Sen. Barbara Boxer says federal regulators might be improperly withholding records on the now-closed San Onofre nuclear power plant.
The California Democrat tells the Nuclear Regulatory Commission that certain documents she requested on a federal investigation at the seaside plant "may have been removed" before they were released to her office.
Boxer heads the Senate Environment and Public Works Committee.
In a letter Thursday she tells NRC head Allison Macfarlane any attempt to obstruct the committee's activities is unacceptable.
The plant between Los Angeles and San Diego was shut down in January 2012 after a small radiation leak led to the discovery of extensive damage to tubing.
Operator Southern California Edison closed the plant for good earlier this year.
The NRC did not immediately respond to a request for comment.