Now we know who did it. D.C. Inspector General Charles J. Willoughby has concluded his 16-month probe of cheating on the D.C. schools’ annual tests by saying that kids, not adults, made the astonishing number of wrong-to-right erasures found on answer sheets.

Never mind that testing companies, academic experts and veteran teachers say that students almost never make more than one or two wrong-to-right erasures per test. Ignore the fact that in Atlanta, where there were similar volumes of erasures on 2009 tests, state investigators with subpoena power found 178 principals and teachers had changed the answers.

DC schools chancellor Kaya Henderson (Matt McClain/The Washington Post)

I had hoped Willoughby’s report would be thorough and independent, since that is what people in such jobs are supposed to be. This thin, biased 14-page document fails egregiously on both counts.

Henderson has not responded to my questions about her involvement in the probe. Deputy Inspector General Blanche L. Bruce said “your assumptions and conclusions are incorrect.” She said her office’s conclusions relied “on the totality of all the evidence.”

Noyes, the only school investigated, had 75 percent of its classrooms flagged by the testing company CTB/McGraw-Hill for unusual numbers of wrong-to-right erasures in 2008, followed by 81 percent in 2009 and 80 percent in 2010, on the D.C. Comprehensive Assessment System tests. At least five Noyes classrooms had wrong-to-right erasure rates of more than 10 per child, while the D.C. average was less than two. (Disclosure: my wife Linda Mathews conceived and supervised a USA Today investigation that revealed 103 D.C. schools had abnormally high erasure rates at least once from 2008 to 2010.)

University of North Carolina-Chapel Hill testing expert Gregory Cizek, a consultant to the Atlanta investigation, told me “nothing we know of” has ever caused such large groups of students to change so many wrong answers to right. Massive erasing only occurs when “others do if for them,” he said.

Yet the Inspector General’s report, quoting no erasure experts, concluded that the D.C. data, without “specific evidence of impropriety . . . was not a sufficient basis to conclude the erasures resulted from cheating.” His investigators interviewed 32 current and former staffers at Noyes and found just one former teacher willing to admit he or she helped some kids get the right answers on one test.

At that point Willoughby let himself be swayed by a powerful official with a vested interest in his conclusions. His investigators could have looked at J.O. Wilson Elementary School, where 93 percent of classrooms were flagged for unusual erasures in 2008, 83 percent in 2009 and 100 percent in 2010. But the report said the investigators discounted those numbers because Henderson told them that Wilson was a great school and “she does not consider a high number of erasures to be an indication of a problem.”

No other schools were visited, the report said, in part because Henderson “revealed no additional evidence to corroborate the allegations.” Was gathering evidence Henderson’s job or Willoughby’s?

D.C. administrators and teachers had an incentive to cheat. They won big bonuses for high test scores. Many were dismissed when scores failed to climb. Still, the Inspector General blamed the students, who got nothing for a good score. The report noted a teacher said the tests were untimed, and students had many opportunities to change their answers. But how could they have been right so often?

The investigators didn’t bother to interview any students about that. None were asked if they remembered making any changes on answer sheets that, when scored, were full of erasures. Investigators did not check Noyes students’ test scores in subsequent years to see if each continued to perform at high levels when test security was tightened and erasures declined.

That is what a real investigator who wanted to get at the truth would have done.