ADD THE Education Department and the District’s U.S. Attorney to the list of those who have found no evidence that D.C. school officials engaged in widespread cheating on state exams. Their findings support the conclusion that emerged from previous investigations: There is no reason to think that systematic cheating was responsible for the improvements in student test scores recorded during the tenure of Schools Chancellor Michelle A. Rhee.

The Education Department’s Office of Inspector General said Monday that its investigation, conducted in tandem with an inquiry by the D.C. inspector general, found no large-scale cheating on standardized tests from 2008 to 2010. Federal officials focused on whether, as alleged by a former school principal in a whistleblower complaint filed under seal in May 2011, results from tampered student tests affected the award of U.S. education dollars. “Unable to substantiate” was the judgment of federal investigators into charges by former Noyes Education Campus principal Adell Cothorne. She said that she interrupted three staff members fiddling with student results on practice tests in November 2010, shortly after Ms. Rhee’s departure, and that her reports to administration officials were ignored.

D.C. Schools Chancellor Kaya Henderson called the claims, repeated this week by Ms. Cothorne in a PBS “Frontline” documentary about Ms. Rhee, “fictitious.” There is no record of the incident having been reported, and school officials named in the account have denied being told about it, Ms. Henderson said in a statement. The legal action filed under the False Claims Act — which both school and PBS officials said they were unaware of until this week — could have benefited Ms. Cothorne, since she would have been entitled to a share of any recovered damages.

Officials question why Ms. Cothorne did not mention the incident to independent investigators probing test integrity when they interviewed her on March 17, 2011, or to investigators from the D.C. Inspector General’s Office who visited the school as part of a 17-month investigation prompted by reports in USA Today about suspect test results. “Do you know how scary the situation is. . . . I didn’t trust anyone,” she told us. She said she didn’t recall being interviewed by the private test-security firm hired by the school system, and even after the heightened attention caused by the USA Today reports, she said she didn’t think anyone would be interested in her story. That strains credulity.

At the same time, questions remain about the higher-than-normal erasure marks at Noyes and the troubling drop in test scores after security was tightened. Cheating could explain both. Even though there have been previous investigations, it’s good that the school system plans to conduct its own examination into the specifics of Ms. Cothorne’s charge. Its conclusions should be made public.

Questions about Noyes and the state test don’t change the larger story about Ms. Rhee’s record. D.C. students made significant progress during her tenure as measured by the National Assessment of Educational Progress (NAEP), a test on which no one has alleged cheating.

Still, those who believe in measuring student success, as we do, have to recognize that as the importance of testing grows, so does the incentive to cheat. If the answer were to eliminate high-stakes testing, there would be no SATs or professional licensing exams. We believe the vast majority of educators would never stoop to tampering with tests. But cheating allegations have to be taken seriously and security protocols put in place. D.C. officials say they have done both, and there is still no evidence to the contrary.