A PBS documentary that aired Tuesday featuring former DC Public School principal Adell Cothorne raises the specter of a possible cover-up. Cothorne alleges that former Chancellor Michelle Rhee and her former deputy, Kaya Henderson. ignored evidence of widespread cheating on high-stakes standardized tests in order to fraudulently collect millions of dollars in federal funding and provide teachers and administrators with $1.5 million in financial incentives.

A 2011 investigative series by USA Today first reported that testing firm CTB/McGraw-Hill found abnormally high levels of wrong-to-right erasures on tests administered to DCPS students between 2008 and 2010. But investigators from the D.C. Office of Inspector General, the U.S. Department of Education and a private consulting firm all assured the public that any cheating was isolated and there was nothing to worry about.

But details revealed on "Frontline" and in Cothorne's previously sealed federal complaint about the inadequacy of those investigations cast new doubt on their conclusions.

In 2006, only 24 percent of students at Noyes Education Campus were proficient in reading and only 10 percent mastered grade-level math. By 2009, "84 percent of Noyes students were proficient in reading and 63 percent in math," earning the staff big bonuses and a dinner at Ruth's Chris Steak House and the school a coveted designation as a Blue Ribbon School. That same year, 80 percent of Noyes' classrooms were tagged for abnormal erasure rates.

Shortly after Cothorne was named principal, guidance counselor John Edwards was assigned to investigate the erasure scandal, but never produced a report. On "Frontline," Cothorne said she personally observed staff members changing test answers in Edwards' office on Nov. 3, 2010. One reportedly told her: "I can't believe a kid drew a spider on his test and I have to erase it." She reported the incident to the central office, but nobody ever followed up by interviewing her. The following spring, after test security was beefed up, scores at Noyes plummeted 25 points.

One agent from Inspector General Charles Willoughby's office assigned to conduct the 17-month probe admitted it was limited to Noyes because Henderson did not provide any more "investigative leads to pursue," and the IG's office "believed news coverage of the scandal would limit future cheating." So, after erecting a firewall around Noyes, further evidence of test tampering at the dozens of other schools flagged by McGraw-Hill was never found because nobody ever bothered to look for it.