When state investigators detailed fraudulent test scores in the Atlanta Public Schools last year, they said some schools had been cheating for years.

The AJC’s new analysis confirms their assertion.

From 2005 to 2010, test scores showed suspicious fluctuations in large numbers of classes — 16 percent to 26 percent. Georgia averaged 5 percent during the same period. Among the nation’s largest school districts, that level of sustained questionable scores was unmatched.

Here’s how the story developed:

2008-2009: The AJC reported statistically unlikely gains in test scores at some Atlanta schools on the Criterion-Referenced Competency Tests.

February 2010: After a state-commissioned erasure analysis flagged 58 Atlanta schools, the state ordered the district to investigate.

August 2010: The state rejected the Atlanta district’s results as inadequate, and then-Gov. Sonny Perdue named two former prosecutors to lead a criminal investigation.

July 2011: The state review determined that cheating occurred in more than half of Atlanta’s elementary and middle schools in 2009. Investigators named about 180 teachers and administrators, some of whom confessed to altering test papers

Now: A Fulton County grand jury is considering criminal charges.

How other area districts fared

DeKalb County: Test scores fluctuated to a high degree in 4 percent to 8 percent of classes each year from 2005 to 2011.

Gwinnett County: The percentage of classes displaying unusual changes ranged from as low as 6 percent in 2010 to as high as 17 percent in 2011. Gains outnumbered decreases each year.

Fulton County: No more than 8 percent of classes showed questionable degrees of change in any year.

Cobb County: Scores in 2 percent to 7 percent of classes changed to unusual degrees each year.